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<sectionid="a-matter-of-style">
<spanid="style"></span><h1><spanclass="section-number">4. </span>A matter of style<aclass="headerlink" href="#a-matter-of-style" title="Link to this heading">¶</a></h1>
<details>
<summary>
Video: why style?</summary><divclass="video_wrapper" style="">
<iframeallowfullscreen="true" src="https://player.vimeo.com/video/499766703" style="border: 0; height: 345px; width: 560px">
</iframe></div><p>Imperial students can also <aclass="reference external" href="https://imperial.cloud.panopto.eu/Panopto/Pages/Viewer.aspx?id=81bda525-60bb-46a7-bdc9-ae1c00da8a74">watch this video on Panopto</a></p>
</details><sectionid="the-value-of-convention">
<h2><spanclass="section-number">4.1. </span>The value of convention<aclass="headerlink" href="#the-value-of-convention" title="Link to this heading">¶</a></h2>
<p>Consider the following definition of the limit of a function <spanclass="math notranslate nohighlight">\(f\)</span> at a
point <spanclass="math notranslate nohighlight">\(c\)</span>:</p>
<divclass="proof proof-type-definition" id="id4">
<divclass="proof-title">
<spanclass="proof-type">Definition 4.1</span>
</div><divclass="proof-content">
<p>Suppose we have a function <spanclass="math notranslate nohighlight">\(f(x): \mathbb{R} \rightarrow \mathbb{R}\)</span>.
Then:</p>
<divclass="math notranslate nohighlight">
\[\lim_{x\rightarrow c} f(x) = L \iff \forall\, \delta > 0,\ \exists\,
\epsilon > 0,\ |x - c| < \epsilon \Rightarrow |f(x) - L| < \delta\]</div>
</div></div><p>This is a perfectly valid definition of a limit: you could use it to go on and
derive all of real analysis. However, you would confuse yourself and your
readers horribly, because I have swapped the roles of <spanclass="math notranslate nohighlight">\(\delta\)</span> and
<spanclass="math notranslate nohighlight">\(\epsilon\)</span> in the definition. The formal properties of mathematical
objects do not depend on the names we give them or, often, on how we lay out
formulae on a page. However, mathematics is designed to be read by humans with
their habit-forming, pattern-matching brains. So if everyone adopts similar
conventions for how to write down mathematics, everyone schooled in those
conventions will find that mathematics easier to understand. This is vitally
important because understanding mathematics is hard. Non-standard notation
makes it doubly hard because the reader has to consciously remember the meaning
of all the symbols. Conversely, once a reader has learned the notational
conventions for a field of mathematics, the meanings of the symbols become
natural and don’t require conscious thought. This leaves all the available
brainpower to concentrate on the mathematical content at hand.</p>
<p>You might at first think that this logic does not apply to computer programs.
After all, a computer program is read by another computer program, and is not
understood but rather acted on mechanically. Surely it doesn’t matter how it
looks or what symbols you use, so long as it’s correct and, possibly, fast?
This entirely understandable sentiment has afflicted almost all programmers at
some point or another and typically has got them into more or less serious
difficulty before they realised that it’s completely wrong.</p>
<p>In reality, a computer program is frequently read. Indeed, code is typically
read many more times than it is written or changed. Most obviously, programmers
read code in order to understand its functionality, and in order to work out
what is wrong with the code when it fails to produce the correct results.
Reading code is very much like reading mathematics because a computer program
is nothing but a realisation of mathematical algorithms. Consequently, the
observation that maths can be very hard to read carries over to code. It is
therefore essential to make code as easy to understand as possible. Another
analogy that carries over from mathematics is that very often it’s one’s own
work that one is trying to understand and correct. This ought to create a very
strong incentive to write very clear code adhering to all the conventions,
because the poor individual who has to read your work to find the bugs might
very well be you!</p>
<p>Just as in mathematics, programming has a whole set of conventions which sit on
top of the formal requirements of a programming language. These exist in order
to make the code easier for all programmers to read. Some of these rules,
typically the more formulaic ones about matters such as code layout and naming
conventions, are somewhat different in different programming languages. Others,
most especially higher level principles like <aclass="reference internal" href="#term-parsimony"><spanclass="xref std std-term">parsimony</span></a> and
<aclass="reference internal" href="#term-modularity"><spanclass="xref std std-term">modularity</span></a>, are universal principles that apply more or less regardless
of the language employed or the sort of programming being undertaken. Good
programming style, like good writing style, is a skill learned through
experience and through receiving feedback on the code you write, and it is not
the intention of this chapter to produce an exhaustive guide. However, it is
useful to introduce some of the key concepts, rules and conventions in a more
formal way.</p>
</section>
<sectionid="pep-8">
<h2><spanclass="section-number">4.2. </span>PEP 8<aclass="headerlink" href="#pep-8" title="Link to this heading">¶</a></h2>
<p>Publishers, journals, and institutions often have style guides
designed to instil a certain uniformity in the use of English (or
other human languages). Similarly, style guides exist for programming
languages. In some languages, the preferred style can vary
significantly from project to project, and there can be vigorous
disagreement between factions about fine points of style, such as
whether or not an opening curly bracket should start on a new
line. Fortunately, the Python community has an essentially unified and
very widely followed set of conventions. These are codified in one of
the Python standards documents, <aclass="reference external" href="https://www.python.org/dev/peps/pep-0008/">PEP 8</a><aclass="footnote-reference brackets" href="#pep" id="id2" role="doc-noteref"><spanclass="fn-bracket">[</span>1<spanclass="fn-bracket">]</span></a>. PEP 8 isn’t all
that long, and it is worth taking the time to read. Not every rule in
PEP 8 is reproduced in this chapter, though many of the most commonly
encountered ones are. Conversely, PEP 8 is rather narrowly concerned
with code layout rules while this chapter roams more widely.</p>
<sectionid="linters">
<h3><spanclass="section-number">4.2.1. </span>Linters<aclass="headerlink" href="#linters" title="Link to this heading">¶</a></h3>
<details>
<summary>
Video: installing and using a linter.</summary><divclass="video_wrapper" style="">
<iframeallowfullscreen="true" src="https://player.vimeo.com/video/499770130" style="border: 0; height: 345px; width: 560px">
</iframe></div><p>Imperial students can also <aclass="reference external" href="https://imperial.cloud.panopto.eu/Panopto/Pages/Viewer.aspx?id=63c09ddd-ec18-4746-b1b9-ae1c00da9680">watch this video on Panopto</a></p>
</details><p>One of the helpful characteristics of PEP 8 is that many of its strictures can
be enforced automatically by a computer program. Programs that automatically
check programming style are called linters. Lint are the little fibres that tend
to stick to clothes, especially suit jackets, and make them ugly. So a linter is
a program that finds the little ugly things in your code.</p>
<p>Originally, you ran the linter with your source file as an input, and it produced
a report as an output, listing all of the problems it found in your code. You
can still run a linter like this, and it’s very useful as an automated check
that the code committed to git is clean. However, Python-aware editors are often
able to run a linter for you, and display the results by highlighting code or
lines of code with problem. The effect is very like the highlighting of spelling
and grammar problems in many word processors and email clients.</p>
<p>One such program is called Flake8. Running Flake8 on all of the source
code in a project, preferably automatically on every commit, is an
excellent mechanism for keeping a project’s code in PEP 8
conformance. Indeed, without a mechanism like this, there is a strong
tendency for programmers to cut style corners, with the effect that
the code in a project becomes harder and harder to read and work with.</p>
</section>
<sectionid="installing-flake8">
<spanid="flake8-extension"></span><h3><spanclass="section-number">4.2.2. </span>Installing Flake8<aclass="headerlink" href="#installing-flake8" title="Link to this heading">¶</a></h3>
<p>Flake8 is a Python package, which can be installed using pip. Make sure you’ve
activated your <aclass="reference internal" href="1_introduction.html#term-virtual-environment"><spanclass="xref std std-term">virtual environment</span></a> and then run:</p>
<divclass="highlight-console notranslate"><divclass="highlight"><pre><span></span><spanclass="gp gp-VirtualEnv">(PoP_venv)</span><spanclass="gp">$ </span>python<spanclass="w"></span>-m<spanclass="w"></span>pip<spanclass="w"></span>install<spanclass="w"></span>flake8
</pre></div>
</div>
<p>This is enough to run Flake8 on the command line, however you will probably want
to set up your editor to highlight PEP 8 incompatibilities in your source. For
Visual Studio Code:</p>
<olclass="arabic simple">
<li><dlclass="simple">
<dt>Install the Flake8 extension:</dt><dd><olclass="loweralpha simple">
<li><p>Click the <imgalt="extensions" src="_images/vscode_extensions.pdf" /> icon on the left of the screen.</p></li>
<li><p>Type <codeclass="xref py py-obj docutils literal notranslate"><spanclass="pre">Flake8</span></code> in the search box.</p></li>
<li><p>Click the blue <codeclass="xref py py-obj docutils literal notranslate"><spanclass="pre">Install</span></code> button.</p></li>
</ol>
</dd>
</dl>
</li>
<li><dlclass="simple">
<dt>Instruct the extension to use Flake8 from your virtual environment:</dt><dd><olclass="loweralpha simple">
<li><p>Type <kbdclass="kbd docutils literal notranslate">control</kbd> + <kbdclass="kbd docutils literal notranslate">,</kbd> (<kbdclass="kbd docutils literal notranslate">⌘</kbd> + <kbdclass="kbd docutils literal notranslate">,</kbd> on Mac) to open the <codeclass="xref py py-obj docutils literal notranslate"><spanclass="pre">Settings</span></code> tab.</p></li>
<li><p>Type <codeclass="xref py py-obj docutils literal notranslate"><spanclass="pre">flake8.importStrategy</span></code> in the <codeclass="xref py py-obj docutils literal notranslate"><spanclass="pre">Search</span><spanclass="pre">settings</span></code> bar at the top of
the page.</p></li>
<li><p>Change the dropdown from <codeclass="xref py py-obj docutils literal notranslate"><spanclass="pre">useBundled</span></code> to <codeclass="xref py py-obj docutils literal notranslate"><spanclass="pre">fromEnvironment</span></code>.</p></li>
</ol>
</dd>
</dl>
</li>
</ol>
<p>The video for this section shows this process.</p>
</section>
<sectionid="how-to-tell-flake8-to-shut-up">
<h3><spanclass="section-number">4.2.3. </span>How to tell Flake8 to shut up<aclass="headerlink" href="#how-to-tell-flake8-to-shut-up" title="Link to this heading">¶</a></h3>
<p>Near the top of PEP 8 is the following heading:</p>
<blockquote>
<div><p>A foolish consistency is the hobgoblin of little minds.</p>
</div></blockquote>
<p>What this refers to is that just religiously following PEP 8 is not enough to
produce highly readable code. Indeed, sometimes the rules might induce you do
to something which makes no sense at all. In those cases, one should step
outside PEP 8. This is a dangerous licence to take, and it is important to
point out that this does not mean that a programmer should ignore PEP 8 merely
because they disagree with a particular convention. Breaking PEP 8 is something
you should do only when you really have to.</p>
<p>In the rare cases where it is necessary to break PEP 8, Flake8 turns into a
problem. It doesn’t know anything of the judgement call that the programmer has
made, and so will complain about the offending code. For example, we have
learned that it is frequently desirable to import names in the
<codeclass="file docutils literal notranslate"><spanclass="pre">__init__.py</span></code> file of a <aclass="reference internal" href="2_programs_in_files.html#term-package"><spanclass="xref std std-term">package</span></a> in order to include them in the
package’s top level <aclass="reference internal" href="2_programs_in_files.html#term-namespace"><spanclass="xref std std-term">namespace</span></a>. The problem with this is that these
names are not used inside the <codeclass="file docutils literal notranslate"><spanclass="pre">__init__.py</span></code> file so Flake8 will complain
that this is an unnecessary import.</p>
<p>One way to suppress linter errors is using a special comment at the end of the
line which causes the error. For example, <codeclass="file docutils literal notranslate"><spanclass="pre">fibonacci/__init__.py</span></code>
contains the following line:</p>
<divclass="highlight-python3 notranslate"><divclass="highlight"><pre><span></span><spanclass="kn">from</span><spanclass="w"></span><spanclass="nn">.fibonacci</span><spanclass="w"></span><spanclass="kn">import</span><spanclass="n">fib</span>
</pre></div>
</div>
<p>This causes the following Flake8 error:</p>
<divclass="highlight-console notranslate"><divclass="highlight"><pre><span></span><spanclass="gp">$ </span>flake8<spanclass="w"></span>fibonacci
<spanclass="go">fibonacci/__init__.py:1:1: F401 '.fibonacci.fib' imported but unused</span>
</pre></div>
</div>
<p>We suppress this error by adding this comment:</p>
<divclass="highlight-python3 notranslate"><divclass="highlight"><pre><span></span><spanclass="kn">from</span><spanclass="w"></span><spanclass="nn">.fibonacci</span><spanclass="w"></span><spanclass="kn">import</span><spanclass="n">fib</span><spanclass="c1"># noqa F401</span>
</pre></div>
</div>
<p>The comment starts with the keyword <codeclass="xref py py-obj docutils literal notranslate"><spanclass="pre">noqa</span></code>, which stands for “no questions
asked” and then gives the error code which is to be ignored for this line. This
can be found in the Flake8 output.</p>
</section>
<sectionid="configuring-flake8">
<h3><spanclass="section-number">4.2.4. </span>Configuring Flake8<aclass="headerlink" href="#configuring-flake8" title="Link to this heading">¶</a></h3>
<p><codeclass="xref py py-obj docutils literal notranslate"><spanclass="pre">noqa</span></code> comments are very useful for one-off suppression of Flake8
errors. However, it’s also likely to be the case that there are some rules that
you just don’t want to apply across your whole project. This configuration can
be achieved in the file <codeclass="file docutils literal notranslate"><spanclass="pre">setup.cfg</span></code>, which lives in the top of your
Git repository alongside <codeclass="file docutils literal notranslate"><spanclass="pre">pyproject.toml</span></code>. <codeclass="file docutils literal notranslate"><spanclass="pre">setup.cfg</span></code> can be
used to configure a lot of different Python tools, so it stores different
configurations in different sections. <aclass="reference internal" href="#flake8conf"><spanclass="std std-numref">Listing 4.1</span></a> shows an example
Flake8 section. The full list of available options is presented in the <aclass="reference external" href="https://flake8.pycqa.org">Flake8
documentation</a>.</p>
<divclass="literal-block-wrapper docutils container" id="id5">
<spanid="flake8conf"></span><divclass="code-block-caption"><spanclass="caption-number">Listing 4.1 </span><spanclass="caption-text">A <codeclass="file docutils literal notranslate"><spanclass="pre">setup.cfg</span></code> Flake8 section which instructs Flake8 to
ignore the <codeclass="file docutils literal notranslate"><spanclass="pre">doc/</span></code> directory, to ignore rule <codeclass="xref py py-obj docutils literal notranslate"><spanclass="pre">D105</span></code> (docstrings
for magic methods), and to add in the non-default rule <codeclass="xref py py-obj docutils literal notranslate"><spanclass="pre">W504</span></code> (error
for a trailing operator at the end of a continued line).</span><aclass="headerlink" href="#id5" title="Link to this code">¶</a></div>
<divclass="highlight-ini notranslate"><divclass="highlight"><pre><span></span><spanclass="k">[flake8]</span>
<spanclass="na">exclude</span><spanclass="w"></span><spanclass="o">=</span><spanclass="w"></span><spanclass="s">doc</span>
<spanclass="na">extend-ignore</span><spanclass="w"></span><spanclass="o">=</span><spanclass="w"></span><spanclass="s">D105</span>
<spanclass="na">extend-select</span><spanclass="w"></span><spanclass="o">=</span><spanclass="w"></span><spanclass="s">W504</span>
</pre></div>
</div>
</div>
</section>
</section>
<sectionid="code-layout">
<h2><spanclass="section-number">4.3. </span>Code layout<aclass="headerlink" href="#code-layout" title="Link to this heading">¶</a></h2>
<p>Perhaps surprisingly, one of the most important factors in making code
readable is the space, or lack of it, between and around the text
which makes up the code. Whitespace affects readability in many
ways. Too much code bunched together makes it hard for the eye to
separate programme statements, while leaving too much space limits the
amount of code which fits in the editor window at once. This requires
the programmer to scroll constantly and to have to remember
definitions which are not currently on the screen.</p>
<p>As in written prose, whitespace can also convey information by
grouping together concepts which are related and separating distinct
ideas. This gives the reader visual clues which can aid in
understanding the code. With this in mind, we now turn to some of the
PEP 8 rules around white space and code formatting.</p>
<sectionid="blank-lines">
<h3><spanclass="section-number">4.3.1. </span>Blank lines<aclass="headerlink" href="#blank-lines" title="Link to this heading">¶</a></h3>
<olclass="arabic simple">
<li><p>Classes and functions defined at the top level of a module
(i.e. not nested in other classes or functions) have two blank
lines before and after them. These are the largest and most
distinct units in a module, so it helps the reader to make them
quite distinct from each other.</p></li>
<li><p>Methods within a class are separated by a single blank
line. Similarly, functions defined inside other functions are
separated from surrounding code by a single blank line.</p></li>
<li><p>Statements within functions usually follow on the immediate next
line, except that logical groups of statements, can be separated by
single blank lines. Think of each statement as a sentence following
on from the previous, with blank lines used to divide the function
into short paragraphs.</p></li>
</ol>
<p><strong>Do not add extra blank lines to space out code</strong>. Vertical space on
the screen is limited, your readers will not thank you.</p>
</section>
<sectionid="white-space-within-lines">
<h3><spanclass="section-number">4.3.2. </span>White space within lines<aclass="headerlink" href="#white-space-within-lines" title="Link to this heading">¶</a></h3>
<olclass="arabic">
<li><p>Don’t put a space after an opening bracket (of any shape), or
before a closing bracket. This is because the role of brackets is
to group their contents, so it’s confusing to visually separate the
bracket from the contents.</p>
<blockquote>
<div><divclass="badcode docutils container">
<divclass="highlight-python3 notranslate"><divclass="highlight"><pre><span></span><spanclass="p">(</span><spanclass="mi">1</span><spanclass="p">,</span><spanclass="mi">2</span><spanclass="p">)</span><spanclass="c1"># Space after opening bracket.</span>
<spanclass="p">(</span><spanclass="mi">1</span><spanclass="p">,</span><spanclass="mi">2</span><spanclass="p">)</span><spanclass="c1"># Space before closing bracket.</span>
</pre></div>
</div>
</div>
<divclass="goodcode docutils container">
<divclass="highlight-python3 notranslate"><divclass="highlight"><pre><span></span><spanclass="p">(</span><spanclass="mi">1</span><spanclass="p">,</span><spanclass="mi">2</span><spanclass="p">)</span><spanclass="c1"># No space between brackets and contents.</span>
</pre></div>
</div>
</div>
</div></blockquote>
</li>
<li><p>Similarly, don’t put a space between the function name and the
opening round bracket of a function call, or between a variable
name and the opening square bracket of an index. In each of these
cases, the opening bracket belongs to the object, so it’s confusing
to insert space between the object name and the bracket.</p>
<blockquote>
<div><divclass="badcode docutils container">
<divclass="highlight-python3 notranslate"><divclass="highlight"><pre><span></span><spanclass="n">sin</span><spanclass="p">(</span><spanclass="mi">1</span><spanclass="p">)</span><spanclass="c1"># Space between function name and bracket.</span>
<spanclass="n">x</span><spanclass="p">[</span><spanclass="mi">0</span><spanclass="p">]</span><spanclass="c1"># Space before index square bracket.</span>
</pre></div>
</div>
</div>
<divclass="goodcode docutils container">
<divclass="highlight-python3 notranslate"><divclass="highlight"><pre><span></span><spanclass="n">sin</span><spanclass="p">(</span><spanclass="mi">1</span><spanclass="p">)</span>
<spanclass="n">x</span><spanclass="p">[</span><spanclass="mi">0</span><spanclass="p">]</span>
</pre></div>
</div>
</div>
</div></blockquote>
</li>
<li><p>Put a space after a comma but not before it, exactly like you would
in writing prose. Following the convention that everyone is used to
from writing aids understanding. Where a trailing comma comes right
before a closing bracket, then don’t put a space. The rule that
there are no spaces before a closing bracket is more important.</p>
<blockquote>
<div><divclass="badcode docutils container">
<divclass="highlight-python3 notranslate"><divclass="highlight"><pre><span></span><spanclass="p">(</span><spanclass="mi">1</span><spanclass="p">,</span><spanclass="mi">2</span><spanclass="p">,</span><spanclass="mi">3</span><spanclass="p">)</span><spanclass="c1"># Spaces missing after commas.</span>
<spanclass="p">(</span><spanclass="mi">1</span><spanclass="p">,</span><spanclass="mi">2</span><spanclass="p">,</span><spanclass="mi">3</span><spanclass="p">)</span><spanclass="c1"># Spurious spaces before commas.</span>
<spanclass="p">(</span><spanclass="mi">1</span><spanclass="p">,</span><spanclass="p">)</span><spanclass="c1"># Space before closing bracket.</span>
</pre></div>
</div>
</div>
<divclass="goodcode docutils container">
<divclass="highlight-python3 notranslate"><divclass="highlight"><pre><span></span><spanclass="p">(</span><spanclass="mi">1</span><spanclass="p">,</span><spanclass="mi">2</span><spanclass="p">,</span><spanclass="mi">3</span><spanclass="p">)</span><spanclass="c1"># Spaces after commas.</span>
<spanclass="p">(</span><spanclass="mi">1</span><spanclass="p">,)</span><spanclass="c1"># No space before closing bracket.</span>
</pre></div>
</div>
</div>
</div></blockquote>
</li>
<li><p>Put exactly one space on each side of an <aclass="reference external" href="https://docs.python.org/3/reference/simple_stmts.html#assignment" title="(in Python v3.14)"><spanclass="xref std std-ref">assignment</span></a> (<codeclass="xref py py-obj docutils literal notranslate"><spanclass="pre">=</span></code>) and an
<aclass="reference external" href="https://docs.python.org/3/reference/simple_stmts.html#augassign" title="(in Python v3.14)"><spanclass="xref std std-ref">augmented assignment</span></a> (<codeclass="xref py py-obj docutils literal notranslate"><spanclass="pre">+=</span></code>, <codeclass="xref py py-obj docutils literal notranslate"><spanclass="pre">-=</span></code>, etc.). In an assignment
statement, the most important distinction is between the left and
right hand sides of the assignment, so adding space here aids the
reader.</p>
<blockquote>
<div><divclass="badcode docutils container">
<divclass="highlight-python3 notranslate"><divclass="highlight"><pre><span></span><spanclass="n">x</span><spanclass="o">=</span><spanclass="mi">1</span><spanclass="c1"># Missing spaces around equals sign.</span>
<spanclass="n">x</span><spanclass="o">+=</span><spanclass="mi">1</span><spanclass="c1"># Missing spaces around augmented addition operator.</span>
<spanclass="n">frog</span><spanclass="o">=</span><spanclass="mi">2</span>
<spanclass="n">cat</span><spanclass="o">=</span><spanclass="mi">3</span><spanclass="c1"># Additional space before equals sign.</span>
</pre></div>
</div>
</div>
<divclass="goodcode docutils container">
<divclass="highlight-python3 notranslate"><divclass="highlight"><pre><span></span><spanclass="n">x</span><spanclass="o">=</span><spanclass="mi">1</span>
<spanclass="n">x</span><spanclass="o">+=</span><spanclass="mi">1</span>
<spanclass="n">frog</span><spanclass="o">=</span><spanclass="mi">2</span>
<spanclass="n">cat</span><spanclass="o">=</span><spanclass="mi">3</span>
</pre></div>
</div>
</div>
</div></blockquote>
</li>
<li><p>Do not put a space either before or after the equals sign of a <aclass="reference external" href="https://docs.python.org/3/tutorial/controlflow.html#tut-keywordargs" title="(in Python v3.14)"><spanclass="xref std std-ref">keyword
argument</span></a>. In this case, grouping the parameter name and
the argument is more important. This rule also creates a visual distinction between
assignment statements and keyword arguments.</p>
<blockquote>
<div><divclass="badcode docutils container">
<divclass="highlight-python3 notranslate"><divclass="highlight"><pre><span></span><spanclass="n">myfunction</span><spanclass="p">(</span><spanclass="n">arg1</span><spanclass="o">=</span><spanclass="n">val1</span><spanclass="p">,</span><spanclass="n">arg2</span><spanclass="o">=</span><spanclass="n">val2</span><spanclass="p">)</span>
</pre></div>
</div>
</div>
<divclass="goodcode docutils container">
<divclass="highlight-python3 notranslate"><divclass="highlight"><pre><span></span><spanclass="n">myfunction</span><spanclass="p">(</span><spanclass="n">arg1</span><spanclass="o">=</span><spanclass="n">val1</span><spanclass="p">,</span><spanclass="n">arg2</span><spanclass="o">=</span><spanclass="n">val2</span><spanclass="p">)</span>
</pre></div>
</div>
</div>
</div></blockquote>
</li>
<li><p>Put exactly one space before and after the lowest priority
mathematical operators in an expression. This has the effect of
visually separating the terms of an expression, as we
conventionally do in mathematics.</p>
<blockquote>
<div><divclass="badcode docutils container">
<divclass="highlight-python3 notranslate"><divclass="highlight"><pre><span></span><spanclass="n">y</span><spanclass="o">=</span><spanclass="mi">3</span><spanclass="o">*</span><spanclass="n">x</span><spanclass="o">**</span><spanclass="mi">2</span><spanclass="o">+</span><spanclass="mi">4</span><spanclass="o">*</span><spanclass="n">x</span><spanclass="o">+</span><spanclass="mi">5</span><spanclass="c1"># No spaces around +</span>
</pre></div>
</div>
</div>
<divclass="goodcode docutils container">
<divclass="highlight-python3 notranslate"><divclass="highlight"><pre><span></span><spanclass="n">y</span><spanclass="o">=</span><spanclass="mi">3</span><spanclass="o">*</span><spanclass="n">x</span><spanclass="o">**</span><spanclass="mi">2</span><spanclass="o">+</span><spanclass="mi">4</span><spanclass="o">*</span><spanclass="n">x</span><spanclass="o">+</span><spanclass="mi">5</span>
</pre></div>
</div>
</div>
</div></blockquote>
</li>
<li><p><strong>Never, ever</strong> have blank spaces at the end of a line, even a blank
line. These tend to get changed by editors, which results in lots
of spurious differences between otherwise identical code. This can
make the difference between two commits of a file very hard to read
indeed.</p></li>
</ol>
</section>
<sectionid="line-breaks">
<h3><spanclass="section-number">4.3.3. </span>Line breaks<aclass="headerlink" href="#line-breaks" title="Link to this heading">¶</a></h3>
<olclass="arabic">
<li><p>Have no lines longer than 79 characters. Limiting the line length
makes lines easier to read, and prevents the editor from
automatically wrapping the line in harder to read ways. Shorter
lines are also very useful when using side-by-side differencing
tools to show the differences between two versions of a piece of
code.</p></li>
<li><p>When breaking lines to fit under 79 characters, it’s better to
break the lines using the implied continuation within round, square
or curly brackets than explicitly with a backslash. This is because
the brackets provide good visual “book ends” for the beginning and
end of the continuation. Of course this is sometimes impossible, so it is
occasionally necessary to use backslashes to break lines.</p></li>
<li><p>When a mathematical operator occurs at a line break, always put the
operator first on the next line, and not last on the first
line. Having the second line start with a mathematical operator
provides a solid visual clue that the next line is a continuation
of the previous line. (If you look closely, this is also the rule
that most publishers of maths books use).</p>
<blockquote>
<div><divclass="badcode docutils container">
<divclass="highlight-python3 notranslate"><divclass="highlight"><pre><span></span><spanclass="n">my_function</span><spanclass="p">(</span><spanclass="n">first_term</span><spanclass="o">+</span><spanclass="c1"># Trailing + operator.</span>
<spanclass="n">second_term</span><spanclass="o">+</span>
<spanclass="n">third_term</span><spanclass="p">)</span>
</pre></div>
</div>
</div>
<divclass="goodcode docutils container">
<divclass="highlight-python3 notranslate"><divclass="highlight"><pre><span></span><spanclass="n">my_function</span><spanclass="p">(</span><spanclass="n">first_term</span>
<spanclass="o">+</span><spanclass="n">second_term</span><spanclass="c1"># Leading + operator</span>
<spanclass="o">+</span><spanclass="n">third_term</span><spanclass="p">)</span>
</pre></div>
</div>
</div>
</div></blockquote>
</li>
</ol>
</section>
<sectionid="indentation">
<h3><spanclass="section-number">4.3.4. </span>Indentation<aclass="headerlink" href="#indentation" title="Link to this heading">¶</a></h3>
<olclass="arabic">
<li><p>Indentation is <em>always</em> by four spaces per indentation level. Usually
Python-aware text editors are good at enforcing this, and this is basically
true of Visual Studio Code. If you’re using a text editor which doesn’t
indent by four spaces (especially if it uses <kbdclass="kbd docutils literal notranslate">tab</kbd> characters for indentation)
then Google how to change it to four spaces!</p></li>
<li><p>When indenting continuation lines inside brackets, there are two
options, usually depending on how many characters are already on
the line before the opening bracket:</p>
<olclass="loweralpha simple">
<li><p>With one or more items on the first line after the opening
bracket. Subsequent lines are indented to one space more than
the opening bracket, so that the first items on each line start
exactly under each other. The closing bracket comes on the same
line as the final item.</p></li>
</ol>
<blockquote>
<div><divclass="goodcode docutils container">
<divclass="highlight-python3 notranslate"><divclass="highlight"><pre><span></span><spanclass="n">capitals</span><spanclass="o">=</span><spanclass="p">{</span><spanclass="s2">"France"</span><spanclass="p">:</span><spanclass="s2">"Paris"</span><spanclass="p">,</span>
<spanclass="s2">"China"</span><spanclass="p">:</span><spanclass="s2">"Beijing"</span><spanclass="p">,</span>
<spanclass="s2">"Australia"</span><spanclass="p">:</span><spanclass="s2">"Canberra"</span><spanclass="p">}</span>
</pre></div>
</div>
</div>
</div></blockquote>
<olclass="loweralpha simple" start="2">
<li><p>With the opening bracket as the last item on the first
line. Subsequent lines are indented more than the first line but
the same as each other. The closing bracket comes on a new line,
and is indented to the same level as the first line.</p></li>
</ol>
<blockquote>
<div><divclass="goodcode docutils container">
<divclass="highlight-python3 notranslate"><divclass="highlight"><pre><span></span><spanclass="n">capitals</span><spanclass="o">=</span><spanclass="p">{</span>
<spanclass="s2">"Central African Republic"</span><spanclass="p">:</span><spanclass="s2">"Bangui"</span><spanclass="p">,</span>
<spanclass="s2">"Trinidad and Tobago"</span><spanclass="p">:</span><spanclass="s2">"Port of Spain"</span><spanclass="p">,</span>
<spanclass="p">}</span>
</pre></div>
</div>
</div>
</div></blockquote>
</li>
</ol>
</section>
</section>
<sectionid="names">
<h2><spanclass="section-number">4.4. </span>Names<aclass="headerlink" href="#names" title="Link to this heading">¶</a></h2>
<p>Programs are full of names. Variables, classes, functions,
modules: much, perhaps most, of the text of a program is made up of
names. The choice of names, therefore, has a massive impact on the
readability of a program. There are two aspects to naming
conventions. One is a set of rules about the formatting of names: when
to use capitals, when underscores and so on. This is covered by PEP 8
and we reproduce some of the important rules below. The second aspect
is the choice of the letter, word, or words that make up a name. This
is much more a matter of judgement, though there are guiding principles
that greatly help with clarity.</p>
<sectionid="pep-8-name-conventions">
<h3><spanclass="section-number">4.4.1. </span>PEP 8 name conventions<aclass="headerlink" href="#pep-8-name-conventions" title="Link to this heading">¶</a></h3>
<p>PEP 8 has some rather detailed rules for naming, including for
advanced cases that we are unlikely to encounter in the short term,
but the most important rules are short and clear:</p>
<dl>
<dt>class names</dt><dd><p>Class names use the CapWords convention: each word in a name is
capitalised and words are concatenated, without underscores between.</p>
<divclass="badcode docutils container">
<divclass="highlight-python3 notranslate"><divclass="highlight"><pre><span></span><spanclass="n">complex_polynomial</span><spanclass="c1"># No capitals, underscore between words.</span>
<spanclass="n">complexPolynomial</span><spanclass="c1"># Missing leading capital.</span>
<spanclass="n">Complex_Polynomial</span><spanclass="c1"># Underscore between words.</span>
</pre></div>
</div>
</div>
<divclass="goodcode docutils container">
<divclass="highlight-python3 notranslate"><divclass="highlight"><pre><span></span><spanclass="n">ComplexPolynomial</span>
</pre></div>
</div>
</div>
</dd>
<dt>exception names</dt><dd><p>Exceptions are classes, so the rules for class names apply with the
addition that exceptions that designate errors should end in
<codeclass="xref py py-obj docutils literal notranslate"><spanclass="pre">Error</span></code>.</p>
<divclass="goodcode docutils container">
<divclass="highlight-python3 notranslate"><divclass="highlight"><pre><span></span><spanclass="n">PolynomialDivisionError</span><spanclass="c1"># For example to indicate indivisibility.</span>
</pre></div>
</div>
</div>
</dd>
<dt>function, variable, and module names</dt><dd><p>Almost all names other than classes are usually written in all
lower case, with underscores separating words. Even proper nouns are
usually spelt with lower case letters to avoid being confused with
class names.</p>
<divclass="badcode docutils container">
<divclass="highlight-python3 notranslate"><divclass="highlight"><pre><span></span><spanclass="k">def</span><spanclass="w"></span><spanclass="nf">Euler</span><spanclass="p">(</span><spanclass="n">n</span><spanclass="p">):</span><spanclass="c1"># Don't capitalise function names.</span>
<spanclass="n">MaxRadius</span><spanclass="o">=</span><spanclass="mf">10.</span><spanclass="c1"># No CamelCase.</span>
</pre></div>
</div>
</div>
<divclass="goodcode docutils container">
<divclass="highlight-python3 notranslate"><divclass="highlight"><pre><span></span><spanclass="k">def</span><spanclass="w"></span><spanclass="nf">euler</span><spanclass="p">(</span><spanclass="n">n</span><spanclass="p">):</span><spanclass="c1"># Lower case, even for names.</span>
<spanclass="n">max_radius</span><spanclass="o">=</span><spanclass="mf">10.</span><spanclass="c1"># Separate words with _.</span>
</pre></div>
</div>
</div>
</dd>
<dt>method parameters</dt><dd><p>The first parameter to an <aclass="reference internal" href="3_objects.html#term-instance-method"><spanclass="xref std std-term">instance method</span></a> is the class
itself. <em>Always and without exception</em> name this parameter <codeclass="xref py py-obj docutils literal notranslate"><spanclass="pre">self</span></code>.</p>
<divclass="badcode docutils container">
<divclass="highlight-python3 notranslate"><divclass="highlight"><pre><span></span><spanclass="k">class</span><spanclass="w"></span><spanclass="nc">MyClass</span><spanclass="p">:</span>
<spanclass="k">def</span><spanclass="w"></span><spanclass="fm">__init__</span><spanclass="p">(</span><spanclass="n">instance</span><spanclass="p">,</span><spanclass="n">arg1</span><spanclass="p">,</span><spanclass="n">arg2</span><spanclass="p">):</span>
<spanclass="o">...</span>
</pre></div>
</div>
</div>
<divclass="goodcode docutils container">
<divclass="highlight-python3 notranslate"><divclass="highlight"><pre><span></span><spanclass="k">class</span><spanclass="w"></span><spanclass="nc">MyClass</span><spanclass="p">:</span>
<spanclass="k">def</span><spanclass="w"></span><spanclass="fm">__init__</span><spanclass="p">(</span><spanclass="bp">self</span><spanclass="p">,</span><spanclass="n">arg1</span><spanclass="p">,</span><spanclass="n">arg2</span><spanclass="p">):</span>
<spanclass="o">...</span>
</pre></div>
</div>
</div>
</dd>
<dt>non-public methods and attributes</dt><dd><p>If a method or attribute is not intended to be directly accessed
from outside the class, it should have a name starting with an
underscore. This provides a clear distinction between the public
interface of a class and its internal implementation.</p>
<divclass="goodcode docutils container">
<divclass="highlight-python3 notranslate"><divclass="highlight"><pre><span></span><spanclass="k">class</span><spanclass="w"></span><spanclass="nc">MyClass</span><spanclass="p">:</span>
<spanclass="k">def</span><spanclass="w"></span><spanclass="nf">_internal_method</span><spanclass="p">(</span><spanclass="bp">self</span><spanclass="p">,</span><spanclass="n">arg1</span><spanclass="p">):</span>
<spanclass="o">...</span>
</pre></div>
</div>
</div>
</dd>
</dl>
</section>
<sectionid="choosing-names">
<h3><spanclass="section-number">4.4.2. </span>Choosing names<aclass="headerlink" href="#choosing-names" title="Link to this heading">¶</a></h3>
<p>Short names help make short lines of code, which in turn makes it easier
to read and understand what the code does to the values it is
operating on. However short names can also be cryptic, making it
difficult to establish what the names mean. This creates a tension:
should names be short to create readable code, or long and descriptive
to clarify their meaning?</p>
<p>A good answer to this dilemma is that local variables should have
short names. These are often the most frequently occurring variables on
a line of code, which makes the statement more
intelligible. Should a reader be unclear what a variable stands for,
the definition of a local variable will not be very far
away. Conversely, a module, class, or function which might be used
far from its definition had better have a descriptive name which makes
its purpose immediately apparent.</p>
</section>
<sectionid="follow-the-mathematics">
<h3><spanclass="section-number">4.4.3. </span>Follow the mathematics<aclass="headerlink" href="#follow-the-mathematics" title="Link to this heading">¶</a></h3>
<p>Remember that the key objective of code style conventions is to make
it easier for readers to understand the code. If the code implements a
mathematical algorithm, then it’s quite likely that readers of that
code will have at least a passing acquaintance with that area of
mathematics. You will therefore greatly help their intuition for what
your code does if the names in the code match the mathematical
conventions for the same concepts. You can use underscores to hint at
subscripts, just like in LaTeX. For example, if you write a function
which changes coordinates, then <codeclass="xref py py-obj docutils literal notranslate"><spanclass="pre">x_old</span></code> and <codeclass="xref py py-obj docutils literal notranslate"><spanclass="pre">x_new</span></code> are likely to be
good names for the coordinate vector before and after the
transformation.</p>
<p>As an exception to the rules about variable case, it is a good idea to
use single capital letter names in circumstances where they would be
used in the maths, for example, to name a matrix.</p>
<p>Mathematicians often use Greek letters as variable names,
occasionally they venture further afield and use Cyrillic or Hebrew
letters. Python does allow for variable names written in other
alphabets, but these are hard to type on many keyboards. Someone
trying to fix bugs in your code will curse you if they can’t even type
the names! Do, by all means, use Greek or other language variable
names where this will make the relationship between the maths and the
code obvious, but write out the Greek letter name in Roman
letters. For example, <codeclass="xref py py-obj docutils literal notranslate"><spanclass="pre">theta</span></code> is a very good name for a variable
representing an angle. Capital Greek letters are sometimes represented
by capitalising the first letter of the Roman word, but take care to
avoid situations where this might be confused for a class name.</p>
</section>
<sectionid="enforcing-name-conventions-in-flake8">
<h3><spanclass="section-number">4.4.4. </span>Enforcing name conventions in Flake8<aclass="headerlink" href="#enforcing-name-conventions-in-flake8" title="Link to this heading">¶</a></h3>
<p>The core Flake8 package does not enforce the PEP 8 naming conventions, but there
is a plugin which does so. Simply install the <codeclass="xref py py-mod docutils literal notranslate"><spanclass="pre">pep8-naming</span></code> package.</p>
<divclass="highlight-console notranslate"><divclass="highlight"><pre><span></span><spanclass="gp">$ </span>python<spanclass="w"></span>-m<spanclass="w"></span>pip<spanclass="w"></span>install<spanclass="w"></span>pep8-naming
</pre></div>
</div>
</section>
</section>
<sectionid="parsimony">
<h2><spanclass="section-number">4.5. </span>Parsimony<aclass="headerlink" href="#parsimony" title="Link to this heading">¶</a></h2>
<p>Good programming style is primarily about making programmes easy to
understand. One of the key limitations of understanding is the sheer
number of objects that the reader can keep in their short term memory
at once. Without diverting into the psychology literature, this is
only a couple of handfuls of values at most. This means that the
largest amount of code that a reader can actively reason about is
limited to a few operations on a few variables. As a programmer, there
are two tools at your disposal to achieve this. The first is to be
parsimonious and not introduce unnecessary temporary variables. The
second is to use abstractions such as classes and function interfaces
to split the problem up into small pieces so that each individual
function or method is small enough for a reader to understand.</p>
<p>As a (somewhat contrived) example, assume that you need to create a list of all
the positive integers less than 9999 which are divisible by all the numbers up
to seven. You could write this in 5 difficult to understand lines:</p>
<divclass="badcode docutils container">
<divclass="highlight-python3 notranslate"><divclass="highlight"><pre><span></span><spanclass="n">result</span><spanclass="o">=</span><spanclass="p">[]</span>
<spanclass="k">for</span><spanclass="n">_</span><spanclass="ow">in</span><spanclass="nb">range</span><spanclass="p">(</span><spanclass="mi">1</span><spanclass="p">,</span><spanclass="mi">9999</span><spanclass="p">):</span>
<spanclass="k">if</span><spanclass="n">_</span><spanclass="o">%</span><spanclass="mi">1</span><spanclass="o">==</span><spanclass="mi">0</span><spanclass="ow">and</span><spanclass="n">_</span><spanclass="o">%</span><spanclass="mi">2</span><spanclass="o">==</span><spanclass="mi">0</span><spanclass="ow">and</span><spanclass="n">_</span><spanclass="o">%</span><spanclass="mi">3</span><spanclass="o">==</span><spanclass="mi">0</span><spanclass="ow">and</span><spanclass="n">_</span><spanclass="o">%</span><spanclass="mi">4</span><spanclass="o">==</span><spanclass="mi">0</span> \
<spanclass="ow">and</span><spanclass="n">_</span><spanclass="o">%</span><spanclass="mi">5</span><spanclass="o">==</span><spanclass="mi">0</span><spanclass="ow">and</span><spanclass="n">_</span><spanclass="o">%</span><spanclass="mi">6</span><spanclass="o">==</span><spanclass="mi">0</span><spanclass="ow">and</span><spanclass="n">_</span><spanclass="o">%</span><spanclass="mi">7</span><spanclass="o">==</span><spanclass="mi">0</span><spanclass="p">:</span>
<spanclass="n">result</span><spanclass="o">.</span><spanclass="n">append</span><spanclass="p">(</span><spanclass="n">_</span><spanclass="p">)</span>
</pre></div>
</div>
</div>
<p>It would be much better to write a single more abstract but simpler line:</p>
<divclass="goodcode docutils container">
<divclass="highlight-python3 notranslate"><divclass="highlight"><pre><span></span><spanclass="n">result</span><spanclass="o">=</span><spanclass="p">[</span><spanclass="n">num</span><spanclass="k">for</span><spanclass="n">num</span><spanclass="ow">in</span><spanclass="nb">range</span><spanclass="p">(</span><spanclass="mi">1</span><spanclass="p">,</span><spanclass="mi">9999</span><spanclass="p">)</span>
<spanclass="k">if</span><spanclass="nb">all</span><spanclass="p">(</span><spanclass="n">num</span><spanclass="o">%</span><spanclass="n">x</span><spanclass="o">==</span><spanclass="mi">0</span><spanclass="k">for</span><spanclass="n">x</span><spanclass="ow">in</span><spanclass="nb">range</span><spanclass="p">(</span><spanclass="mi">1</span><spanclass="p">,</span><spanclass="mi">8</span><spanclass="p">))]</span>
</pre></div>
</div>
</div>
<sectionid="use-comprehensions">
<h3><spanclass="section-number">4.5.1. </span>Use comprehensions<aclass="headerlink" href="#use-comprehensions" title="Link to this heading">¶</a></h3>
<p>It is very common to write loops to populate collection objects with
values. For example, we might make a list of the first 10 square
numbers for further use:</p>
<divclass="badcode docutils container">
<divclass="highlight-python3 notranslate"><divclass="highlight"><pre><span></span><spanclass="n">squares</span><spanclass="o">=</span><spanclass="p">[]</span>
<spanclass="k">for</span><spanclass="n">i</span><spanclass="ow">in</span><spanclass="nb">range</span><spanclass="p">(</span><spanclass="mi">10</span><spanclass="p">):</span>
<spanclass="n">squares</span><spanclass="o">.</span><spanclass="n">append</span><spanclass="p">((</span><spanclass="n">i</span><spanclass="o">+</span><spanclass="mi">1</span><spanclass="p">)</span><spanclass="o">**</span><spanclass="mi">2</span><spanclass="p">)</span>
</pre></div>
</div>
</div>
<p>This is a fairly typical, if simple, example. It takes three lines of
code: one to initialise the list, one to loop, and one to add the
values to the list. Alternatively, if we had used a <aclass="reference external" href="https://docs.python.org/3/tutorial/datastructures.html#tut-listcomps" title="(in Python v3.14)"><spanclass="xref std std-ref">list
comprehension</span></a>, all three steps would have been subsumed into a single
operation:</p>
<divclass="goodcode docutils container">
<divclass="highlight-python3 notranslate"><divclass="highlight"><pre><span></span><spanclass="n">squares</span><spanclass="o">=</span><spanclass="p">[(</span><spanclass="n">i</span><spanclass="o">+</span><spanclass="mi">1</span><spanclass="p">)</span><spanclass="o">**</span><spanclass="mi">2</span><spanclass="k">for</span><spanclass="n">i</span><spanclass="ow">in</span><spanclass="nb">range</span><spanclass="p">(</span><spanclass="mi">10</span><spanclass="p">)]</span>
</pre></div>
</div>
</div>
<p>At least for fairly simple operations, comprehensions are almost
always easier for the reader to understand than loops. In addition to
lists, comprehensions are also available for <aclass="reference external" href="https://docs.python.org/3/tutorial/datastructures.html#tut-sets" title="(in Python v3.14)"><spanclass="xref std std-ref">sets</span></a>
and <aclass="reference external" href="https://docs.python.org/3/tutorial/datastructures.html#tut-dictionaries" title="(in Python v3.14)"><spanclass="xref std std-ref">dictionaries</span></a>.</p>
</section>
<sectionid="redundant-logical-expressions">
<h3><spanclass="section-number">4.5.2. </span>Redundant logical expressions<aclass="headerlink" href="#redundant-logical-expressions" title="Link to this heading">¶</a></h3>
<p>One exceptionally common failure of parsimony is to write expressions of the following form:</p>
<divclass="badcode docutils container">
<divclass="highlight-python3 notranslate"><divclass="highlight"><pre><span></span><spanclass="k">if</span><spanclass="n">var</span><spanclass="o">==</span><spanclass="kc">True</span><spanclass="p">:</span>
</pre></div>
</div>
</div>
<p>To see the problem with this statement, let’s write out its truth table:</p>
<tableclass="center-align-center-col docutils align-default">
<thead>
<trclass="row-odd"><thclass="head"><p><codeclass="xref py py-obj docutils literal notranslate"><spanclass="pre">var</span></code></p></th>
<thclass="head"><p><codeclass="xref py py-obj docutils literal notranslate"><spanclass="pre">var</span><spanclass="pre">==</span><spanclass="pre">True</span></code></p></th>
</tr>
</thead>
<tbody>
<trclass="row-even"><td><p>T</p></td>
<td><p>T</p></td>
</tr>
<trclass="row-odd"><td><p>F</p></td>
<td><p>F</p></td>
</tr>
</tbody>
</table>
<p>In other words, the expressions <codeclass="xref py py-obj docutils literal notranslate"><spanclass="pre">var</span></code> and <codeclass="xref py py-obj docutils literal notranslate"><spanclass="pre">var</span><spanclass="pre">==</span><spanclass="pre">True</span></code> are logically
equivalent (at least assuming <codeclass="xref py py-obj docutils literal notranslate"><spanclass="pre">var</span></code> is a <aclass="reference external" href="https://docs.python.org/3/library/stdtypes.html#bltin-boolean-values" title="(in Python v3.14)"><spanclass="xref std std-ref">boolean value</span></a>), so it would
have been more parsimonious to write:</p>
<divclass="goodcode docutils container">
<divclass="highlight-python3 notranslate"><divclass="highlight"><pre><span></span><spanclass="k">if</span><spanclass="n">var</span><spanclass="p">:</span>
</pre></div>
</div>
</div>
<p>Similarly:</p>
<divclass="badcode docutils container">
<divclass="highlight-python3 notranslate"><divclass="highlight"><pre><span></span><spanclass="k">if</span><spanclass="n">var</span><spanclass="o">==</span><spanclass="kc">False</span><spanclass="p">:</span>
</pre></div>
</div>
</div>
<p>is frowned upon by programmers in favour of:</p>
<divclass="goodcode docutils container">
<divclass="highlight-python3 notranslate"><divclass="highlight"><pre><span></span><spanclass="k">if</span><spanclass="ow">not</span><spanclass="n">var</span><spanclass="p">:</span>
</pre></div>
</div>
</div>
<p>Finally, the use of <aclass="reference external" href="https://docs.python.org/3/reference/compound_stmts.html#else" title="(in Python v3.14)"><spanclass="xref std std-ref">else</span></a> (or <aclass="reference external" href="https://docs.python.org/3/reference/compound_stmts.html#elif" title="(in Python v3.14)"><spanclass="xref std std-ref">elif</span></a>) can reduce the number
of logical expressions that the reader has to read and
understand. This means that:</p>
<divclass="badcode docutils container">
<divclass="highlight-python3 notranslate"><divclass="highlight"><pre><span></span><spanclass="k">if</span><spanclass="n">var</span><spanclass="p">:</span>
<spanclass="c1"># Some code</span>
<spanclass="k">if</span><spanclass="ow">not</span><spanclass="n">var</span><spanclass="p">:</span>
<spanclass="c1"># Some other code</span>
</pre></div>
</div>
</div>
<p>should be avoided in favour of:</p>
<divclass="goodcode docutils container">
<divclass="highlight-python3 notranslate"><divclass="highlight"><pre><span></span><spanclass="k">if</span><spanclass="n">var</span><spanclass="p">:</span>
<spanclass="c1"># Some code</span>
<spanclass="k">else</span><spanclass="p">:</span>
<spanclass="c1"># Some other code.</span>
</pre></div>
</div>
</div>
<p>In addition to having fewer logical operations which the reader needs
to understand, the <codeclass="xref py py-obj docutils literal notranslate"><spanclass="pre">if...else</span></code> version explicitly ties
the two cases together as alternatives, which is an additional aid to
comprehension.</p>
</section>
<sectionid="use-the-fact-that-every-object-is-true-or-false">
<h3><spanclass="section-number">4.5.3. </span>Use the fact that every object is True or False<aclass="headerlink" href="#use-the-fact-that-every-object-is-true-or-false" title="Link to this heading">¶</a></h3>
<p>Every Python object is logically either <aclass="reference external" href="https://docs.python.org/3/library/constants.html#True" title="(in Python v3.14)"><codeclass="xref py py-data docutils literal notranslate"><spanclass="pre">True</span></code></a> or <aclass="reference external" href="https://docs.python.org/3/library/constants.html#False" title="(in Python v3.14)"><codeclass="xref py py-data docutils literal notranslate"><spanclass="pre">False</span></code></a> according to the
following rules:</p>
<olclass="arabic simple">
<li><p><aclass="reference external" href="https://docs.python.org/3/library/constants.html#None" title="(in Python v3.14)"><codeclass="xref py py-data docutils literal notranslate"><spanclass="pre">None</span></code></a> is <aclass="reference external" href="https://docs.python.org/3/library/constants.html#False" title="(in Python v3.14)"><codeclass="xref py py-data docutils literal notranslate"><spanclass="pre">False</span></code></a>.</p></li>
<li><p>Zero is <aclass="reference external" href="https://docs.python.org/3/library/constants.html#False" title="(in Python v3.14)"><codeclass="xref py py-data docutils literal notranslate"><spanclass="pre">False</span></code></a>, all other numerical values are <aclass="reference external" href="https://docs.python.org/3/library/constants.html#True" title="(in Python v3.14)"><codeclass="xref py py-data docutils literal notranslate"><spanclass="pre">True</span></code></a>.</p></li>
<li><p>An empty collection is <aclass="reference external" href="https://docs.python.org/3/library/constants.html#False" title="(in Python v3.14)"><codeclass="xref py py-data docutils literal notranslate"><spanclass="pre">False</span></code></a>, any other container is <aclass="reference external" href="https://docs.python.org/3/library/constants.html#True" title="(in Python v3.14)"><codeclass="xref py py-data docutils literal notranslate"><spanclass="pre">True</span></code></a>. For
example, an empty list is <aclass="reference external" href="https://docs.python.org/3/library/constants.html#False" title="(in Python v3.14)"><codeclass="xref py py-data docutils literal notranslate"><spanclass="pre">False</span></code></a>, but the list <codeclass="xref py py-obj docutils literal notranslate"><spanclass="pre">[0,</span><spanclass="pre">0]</span></code> is <aclass="reference external" href="https://docs.python.org/3/library/constants.html#True" title="(in Python v3.14)"><codeclass="xref py py-data docutils literal notranslate"><spanclass="pre">True</span></code></a>.</p></li>
<li><p>The null string <codeclass="xref py py-obj docutils literal notranslate"><spanclass="pre">""</span></code> is <aclass="reference external" href="https://docs.python.org/3/library/constants.html#False" title="(in Python v3.14)"><codeclass="xref py py-data docutils literal notranslate"><spanclass="pre">False</span></code></a>, a string containing any characters is <aclass="reference external" href="https://docs.python.org/3/library/constants.html#True" title="(in Python v3.14)"><codeclass="xref py py-data docutils literal notranslate"><spanclass="pre">True</span></code></a>.</p></li>
<li><p>A user-defined class is <aclass="reference external" href="https://docs.python.org/3/library/constants.html#True" title="(in Python v3.14)"><codeclass="xref py py-data docutils literal notranslate"><spanclass="pre">True</span></code></a> unless:</p>
<olclass="loweralpha simple">
<li><p>It defines the <aclass="reference external" href="https://docs.python.org/3/reference/datamodel.html#object.__bool__" title="(in Python v3.14)"><codeclass="xref py py-meth docutils literal notranslate"><spanclass="pre">__bool__()</span></code></a><aclass="reference internal" href="3_objects.html#term-special-method"><spanclass="xref std std-term">special
method</span></a>. In this case the truth value is whatever this method
returns.</p></li>
<li><p>It doesn’t define <aclass="reference external" href="https://docs.python.org/3/reference/datamodel.html#object.__bool__" title="(in Python v3.14)"><codeclass="xref py py-meth docutils literal notranslate"><spanclass="pre">__bool__()</span></code></a> but does define
<aclass="reference external" href="https://docs.python.org/3/reference/datamodel.html#object.__len__" title="(in Python v3.14)"><codeclass="xref py py-meth docutils literal notranslate"><spanclass="pre">__len__()</span></code></a>. In this case the object is <aclass="reference external" href="https://docs.python.org/3/library/constants.html#False" title="(in Python v3.14)"><codeclass="xref py py-data docutils literal notranslate"><spanclass="pre">False</span></code></a> if the
length is zero, and <aclass="reference external" href="https://docs.python.org/3/library/constants.html#True" title="(in Python v3.14)"><codeclass="xref py py-data docutils literal notranslate"><spanclass="pre">True</span></code></a> otherwise.</p></li>
</ol>
</li>
</ol>
<p>These rules are laid out formally in <aclass="reference external" href="https://docs.python.org/3/library/stdtypes.html#truth" title="(in Python v3.14)"><spanclass="xref std std-ref">the Python documentation</span></a>. One way that they can be used to write simpler, clearer code
is in the very common case of code that should only execute if a
collection object actually contains something. In that case, this form
of test is to be preferred:</p>
<divclass="goodcode docutils container">
<divclass="highlight-python3 notranslate"><divclass="highlight"><pre><span></span><spanclass="k">if</span><spanclass="n">mysequence</span><spanclass="p">:</span>
<spanclass="c1"># Some code using mysequence</span>
</pre></div>
</div>
</div>
<p>instead of:</p>
<divclass="badcode docutils container">
<divclass="highlight-python3 notranslate"><divclass="highlight"><pre><span></span><spanclass="k">if</span><spanclass="nb">len</span><spanclass="p">(</span><spanclass="n">mysequence</span><spanclass="p">)</span><spanclass="o">></span><spanclass="mi">0</span><spanclass="p">:</span>
<spanclass="c1"># Some code using mysequence</span>
</pre></div>
</div>
</div>
</section>
<sectionid="avoid-repetition">
<spanid="repetition"></span><h3><spanclass="section-number">4.5.4. </span>Avoid repetition<aclass="headerlink" href="#avoid-repetition" title="Link to this heading">¶</a></h3>
<p>Programmers very frequently need to do <em>nearly</em> the same thing over and over.
One obvious way to do this is to write code for the first case, then copy and
paste the code for subsequent cases, making changes as required. There are a
number of significant problems with this approach. First, it multiplies the
amount of code that a reader has to understand, and does so in a particularly
pernicious way. A reader will effectively have to play “spot the difference”
between the different code versions, and hope they don’t miss something. Second,
it makes it incredibly easy for to get confused about which version of the code
a programmer is supposed to be working on. There are few things more frustrating
than attempting to fix a bug and repeatedly seeing that nothing changes, only to
discover hours (or days) later that you have been working on the wrong piece of
nearly-identical code. Finally, lets suppose that a bug is fixed - what happens
to the near-identical clones of that code? The chance is very high that the bug
stays unfixed in those versions thereby creating yet another spot the difference
puzzle for the next person encountering a bug.</p>
<p>Abstractions are essentially tools for removing harmful repetition. For
example, it may be possible to bundle up the repeated code in a function or
class, and to encode the differences between versions in the <aclass="reference external" href="https://docs.python.org/3/glossary.html#term-parameter" title="(in Python v3.14)"><spanclass="xref std std-term">parameters</span></a> to the function or class constructor. If the differences between
the versions of the code require different code, as opposed to different values
of some quantities, then it may be possible to use <aclass="reference internal" href="7_inheritance.html#term-inheritance"><spanclass="xref std std-term">inheritance</span></a> to avoid
repetition. We will return to this in <aclass="reference internal" href="7_inheritance.html#inheritance"><spanclass="std std-numref">Chapter 7</span></a>.</p>
</section>
</section>
<sectionid="comments">
<h2><spanclass="section-number">4.6. </span>Comments<aclass="headerlink" href="#comments" title="Link to this heading">¶</a></h2>
<p>Comments are non-code text included in programs to help explain what
they do. Since comments exist to aid understanding, some programmers
come to the conclusion that more comments imply more
understanding. Indeed, some programmers are even taught that every
line of code should have a comment. This could not be more wrong!</p>
<p>While judiciously deployed comments can be an essential aid to
understanding, too many comments can be worse than too few. If the
code is simple, elegant, and closely follows how a reader would expect
the algorithm to be written, then it will be readily understood
without comments. Conversely, attempting to rescue obscure, badly
thought-through code by writing about it is unlikely to remedy the
situation.</p>
<p>A further severe problem with comments is that they can easily become
out of date. If a piece of code is modified, it is all too easy for the
programmer to neglect to update accompanying comments. The result is
comments which explain one thing, code which does something else, and
exceptionally baffled readers.</p>
<sectionid="three-rules-for-commenting">
<h3><spanclass="section-number">4.6.1. </span>Three rules for commenting<aclass="headerlink" href="#three-rules-for-commenting" title="Link to this heading">¶</a></h3>
<olclass="arabic simple">
<li><p>If code is so simple, clear, and obvious that it can be easily
understood without comments, don’t comment.</p></li>
<li><p>If code is not easily understood without comments, the problem is
probably the code. Refactor the code to be simpler and easier to
understand.</p></li>
<li><p>If, and only if, you are convinced that it is strictly necessary to do
something unobvious, then do so and include a comment.</p></li>
</ol>
</section>
<sectionid="comment-why-not-what">
<h3><spanclass="section-number">4.6.2. </span>Comment why, not what<aclass="headerlink" href="#comment-why-not-what" title="Link to this heading">¶</a></h3>
<p>Even where a comment is unavoidable, it should still usually be
obvious <em>what</em> it is that code does. It is far more likely to be
justifiable to include a comment about <em>why</em> a particular approach is
taken. For example, it might be worth commenting why an apparently
simpler alternative strategy is actually invalid.</p>
</section>
<sectionid="pep-8-rules-for-comments">
<h3><spanclass="section-number">4.6.3. </span>PEP 8 rules for comments<aclass="headerlink" href="#pep-8-rules-for-comments" title="Link to this heading">¶</a></h3>
<p>Comments start with a single <codeclass="file docutils literal notranslate"><spanclass="pre">#</span></code> followed by a single space.
<aclass="reference internal" href="#term-inline-comment"><spanclass="xref std std-term">Inline comments</span></a> are separated from the code by at
least two spaces.</p>
<divclass="badcode docutils container">
<divclass="highlight-python3 notranslate"><divclass="highlight"><pre><span></span><spanclass="bp">self</span><spanclass="o">.</span><spanclass="n">count</span><spanclass="o">+=</span><spanclass="mi">1</span><spanclass="c1"># No space between code and comment.</span>
<spanclass="bp">self</span><spanclass="o">.</span><spanclass="n">count</span><spanclass="o">+=</span><spanclass="mi">1</span><spanclass="c1">#No space between # and comment text.</span>
</pre></div>
</div>
</div>
<divclass="goodcode docutils container">
<divclass="highlight-python3 notranslate"><divclass="highlight"><pre><span></span><spanclass="bp">self</span><spanclass="o">.</span><spanclass="n">count</span><spanclass="o">+=</span><spanclass="mi">1</span><spanclass="c1"># Two spaces before #, one after.</span>
</pre></div>
</div>
</div>
<p>Each line of a block comment starts with a single <codeclass="file docutils literal notranslate"><spanclass="pre">#</span></code> indented to the
same level as a normal line of code. The <codeclass="file docutils literal notranslate"><spanclass="pre">#</span></code> is followed by a single
space, unless a particular piece of comment should be indented with respect to
the paragraph it is in, in which case additional spaces are allowed.</p>
<divclass="goodcode docutils container">
<divclass="highlight-python3 notranslate"><divclass="highlight"><pre><span></span><spanclass="k">if</span><spanclass="n">somecondition</span><spanclass="p">(</span><spanclass="n">data</span><spanclass="p">):</span>
<spanclass="c1"># Comment indented to the same level as the contents of the if</span>
<spanclass="c1"># block.</span>
</pre></div>
</div>
</div>
</section>
</section>
<sectionid="docstrings">
<h2><spanclass="section-number">4.7. </span>Docstrings<aclass="headerlink" href="#docstrings" title="Link to this heading">¶</a></h2>
<p>There is one enormous exception to the rule that comments should be
used only sparingly: docstrings. Docstrings (a portmanteau of
“documentation strings”) are comments at the start of modules,
classes, and functions which describe public interfaces. The entire
point of a public interface is that the programmer using it should not
have to concern themselves with how it is implemented. They should,
therefore, not need to read the code in order to understand how to use
it.</p>
<p>The <aclass="reference internal" href="2_programs_in_files.html#term-Python-interpreter"><spanclass="xref std std-term">Python interpreter</span></a> has special support for docstrings. When a user
calls <aclass="reference external" href="https://docs.python.org/3/library/functions.html#help" title="(in Python v3.14)"><codeclass="xref py py-func docutils literal notranslate"><spanclass="pre">help()</span></code></a> on an <aclass="reference external" href="https://docs.python.org/3/glossary.html#term-object" title="(in Python v3.14)"><spanclass="xref std std-term">object</span></a> (including a function or <aclass="reference internal" href="3_objects.html#term-method"><spanclass="xref std std-term">method</span></a>)
then any docstring on that object is used as the body of the resulting help
message. Docstrings are also understood by the Python documentation generation
system, <aclass="reference external" href="https://www.sphinx-doc.org/en/master/">Sphinx</a>. This enables
documentation webpages to be automatically generated from Python code. The
Python documentation in this course is generated by this system. For example,
recall that we met the function <aclass="reference internal" href="fibonacci.html#fibonacci.fibonacci.fib" title="fibonacci.fibonacci.fib"><codeclass="xref py py-func docutils literal notranslate"><spanclass="pre">fib()</span></code></a>, which
calculates Fibonacci numbers, in <aclass="reference internal" href="2_programs_in_files.html#modules"><spanclass="std std-numref">Section 2.3</span></a>. We can ask
<aclass="reference internal" href="fibonacci.html#fibonacci.fibonacci.fib" title="fibonacci.fibonacci.fib"><codeclass="xref py py-func docutils literal notranslate"><spanclass="pre">fib()</span></code></a> for its documentation:</p>
<divclass="highlight-ipython3 notranslate"><divclass="highlight"><pre><span></span><spanclass="gp">In [1]: </span><spanclass="kn">import</span><spanclass="w"></span><spanclass="nn">fibonacci</span>
<spanclass="gp">In [2]: </span><spanclass="n">help</span><spanclass="p">(</span><spanclass="n">fibonacci</span><spanclass="o">.</span><spanclass="n">fib</span><spanclass="p">)</span>
</pre></div>
</div>
<p>The following is displayed:</p>
<blockquote>
<div><p>Help on function fib in module fibonacci.fibonacci:</p>
<dlclass="simple">
<dt>fib(n)</dt><dd><p>Return the n-th Fibonacci number.</p>
</dd>
</dl>
</div></blockquote>
<p>There is also a specific IPython help extension, which also works in Jupyter
notebooks (IPython and Jupyter are related projects). Appending a question mark
<kbdclass="kbd docutils literal notranslate">?</kbd> to an object name prints a slightly different version of the help
information:</p>
<divclass="highlight-ipython3 notranslate"><divclass="highlight"><pre><span></span><spanclass="gp">In [3]: </span>fibonacci.fib<spanclass="o">?</span>
<spanclass="go">Signature: fibonacci.fib(n)</span>
<spanclass="go">Docstring: Return the n-th Fibonacci number.</span>
<spanclass="go">File: ~/docs/principles_of_programming/object-oriented-programming/fibonacci/fibonacci.py</span>
<spanclass="go">Type: function</span>
</pre></div>
</div>
<p>Finally, the same information is used in the <aclass="reference internal" href="fibonacci.html#fibonacci.fibonacci.fib" title="fibonacci.fibonacci.fib"><codeclass="xref py py-func docutils literal notranslate"><spanclass="pre">web</span><spanclass="pre">documentation</span></code></a>. Notice that the <aclass="reference internal" href="#term-function-signature"><spanclass="xref std std-term">function signature</span></a> is
not a part of the docstring. Python is capable of extracting the signature
of the function and adding it into the documentation without the programmer
having to manually add it to the docstring.</p>
<sectionid="where-to-use-docstrings">
<h3><spanclass="section-number">4.7.1. </span>Where to use docstrings<aclass="headerlink" href="#where-to-use-docstrings" title="Link to this heading">¶</a></h3>
<p>Every public module, class, function, and method should have a docstring.
“Public” in this context means any code which is intended to be accessed from
outside the <aclass="reference internal" href="2_programs_in_files.html#modules"><spanclass="std std-ref">module</span></a> in which it is defined.</p>
</section>
<sectionid="docstring-conventions">
<h3><spanclass="section-number">4.7.2. </span>Docstring conventions<aclass="headerlink" href="#docstring-conventions" title="Link to this heading">¶</a></h3>
<p>Python itself doesn’t know anything about docstring contents, it will simply
display the docstring when you ask for help. However, other tools such as those
that generate websites from documentation depend on you following the
conventions.</p>
<p>By convention, docstrings are delimited by three double quote characters (<codeclass="xref py py-obj docutils literal notranslate"><spanclass="pre">"""</span></code>).</p>
</section>
<sectionid="short-docstrings">
<h3><spanclass="section-number">4.7.3. </span>Short docstrings<aclass="headerlink" href="#short-docstrings" title="Link to this heading">¶</a></h3>
<p>Simple functions which take one or two arguments can be documented with a single
line docstring which simply says what the function does. The Fibonacci example above