python.md

title	Python
date	2020-12-23 10:41:20 -0800
icon	icon-python
background	bg-blue-600
tags	script interpret
categories	Programming
intro	The [Python](https://www.python.org/) cheat sheet is a one-page reference sheet for the Python 3 programming language.

Getting Started {.cols-3}

Introduction

• Python(python.org)
• Learn X in Y minutes(learnxinyminutes.com)
• Regex in python(quickref.me)

Hello World

>>>print("Hello, World!")
Hello, World!

The famous "Hello World" program in Python

Variables

x=4# x is of type intx="Sally"# x is now of type strprint(x)

Python has no command for declaring a variable.

Data Types {.row-span-2}

str	Text
int, float, complex	Numeric
list, tuple, range	Sequence
dict	Mapping
set, frozenset	Set
bool	Boolean
bytes, bytearray, memoryview	Binary
See: Data Types

Slicing String

>>>b="Hello, World!">>>print(b[2:5])
llo

See: Strings

Lists

mylist= []
mylist.append(1)
mylist.append(2)
forxinmylist:
print(x) # prints out 1,2

See: Lists

If Else

a=200ifa>0:
print("a is greater than 0")
else:
print("a is not greater than 0")

See: Flow control

Loops

forxinrange(6):
ifx==3: breakprint(x)
else:
print("Finally finished!")

See: Loops

Functions

>>>defmy_function():
... print("Hello from a function")
...
>>>my_function()
Hellofromafunction

See: Functions

File Handling {.col-span-2}

withopen("myfile.txt", "r", encoding='utf8') asfile:
forxinfile:
print(x)

See: File Handling

Arithmetic

result=10+30# => 40result=40-10# => 30result=50*5# => 250result=16/4# => 4result=25%2# => 1result=5**3# => 125

Plus-Equals

counter=0counter+=10# => 10counter=0counter=counter+10# => 10message="Part 1."# => Part 1.Part 2.message+="Part 2."

Python Data Types {.cols-3}

Strings

s="Hello World"s='Hello World'a="""Multiline StringsLorem ipsum dolor sit amet,consectetur adipiscing elit """

See: Strings

Numbers

x=1# inty=2.8# floatz=1j# complex>>>print(type(x))
<class'int'>

Booleans

a=Trueb=Falsebool(0) # => Falsebool(1) # => True

Lists

list1= ["apple", "banana", "cherry"]
list2= [True, False, False]
list3= [1, 5, 7, 9, 3]
list4=list((1, 5, 7, 9, 3))

See: Lists

Tuple

a= (1, 2, 3)
a=tuple((1, 2, 3))

Similar to List but immutable

Set

set1= {"a", "b", "c"} set2=set(("a", "b", "c"))

Set of unique items/objects

Dictionary

>>>empty_dict= {}
>>>a= {"one": 1, "two": 2, "three": 3}
>>>a["one"]
1>>>a.keys()
dict_keys(['one', 'two', 'three'])
>>>a.values()
dict_values([1, 2, 3])
>>>a.update({"four": 4})
>>>a.keys()
dict_keys(['one', 'two', 'three', 'four'])
>>>a['four']
4

Key: Value pair, JSON like object

Casting

Integers

x=int(1) # x will be 1y=int(2.8) # y will be 2z=int("3") # z will be 3

Floats

x=float(1) # x will be 1.0y=float(2.8) # y will be 2.8z=float("3") # z will be 3.0w=float("4.2") # w will be 4.2

Strings

x=str("s1") # x will be 's1'y=str(2) # y will be '2'z=str(3.0) # z will be '3.0'

Python Strings {.cols-3}

Array-like

>>>a="Hello, World">>>print(a[1])
e>>>print(a[len(a)-1])
d

Get the character at position 1

Looping

>>>forxin"abc":
... print(x)
abc

Loop through the letters in the word "banana"

Slicing string {.row-span-4}

 ┌───┬───┬───┬───┬───┬───┬───┐
| m | y | b | a | c | o | n |
└───┴───┴───┴───┴───┴───┴───┘
01234567
-7 -6 -5 -4 -3 -2 -1

---

>>>s='mybacon'>>>s[2:5]
'bac'>>>s[0:2]
'my'

>>>s='mybacon'>>>s[:2]
'my'>>>s[2:]
'bacon'>>>s[:2] +s[2:]
'mybacon'>>>s[:]
'mybacon'

>>>s='mybacon'>>>s[-5:-1]
'baco'>>>s[2:6]
'baco'

With a stride

>>>s='12345'*5>>>s'1234512345123451234512345'>>>s[::5]
'11111'>>>s[4::5]
'55555'>>>s[::-5]
'55555'>>>s[::-1]
'5432154321543215432154321'

String Length

>>>a="Hello, World!">>>print(len(a))
13

The len() function returns the length of a string

Multiple copies

>>>s='===+'>>>n=8>>>s*n'===+===+===+===+===+===+===+===+'

Check String

>>>s='spam'>>>sin'I saw spamalot!'True>>>snotin'I saw The Holy Grail!'True

Concatenates

>>>s='spam'>>>t='egg'>>>s+t'spamegg'>>>'spam''egg''spamegg'

Formatting {.col-span-2}

name="John"print("Hello, %s!"%name)

name="John"age=23print("%s is %d years old."% (name, age))

format() Method

txt1="My name is {fname}, I'm {age}".format(fname="John", age=36)
txt2="My name is {0}, I'm {1}".format("John",36)
txt3="My name is {}, I'm {}".format("John",36)

Input

>>>name=input("Enter your name: ")
Enteryourname: Tom>>>name'Tom'

Get input data from console

Join

>>>"#".join(["John", "Peter", "Vicky"])
'John#Peter#Vicky'

Endswith

>>>"Hello, world!".endswith("!")
True

Python Lists {.cols-3}

Defining

>>>li1= []
>>>li1
[]
>>>li2= [4, 5, 6]
>>>li2
[4, 5, 6]
>>>li3=list((1, 2, 3))
>>>li3
[1, 2, 3]
>>>li4=list(range(1, 11))
>>>li4
[1, 2, 3, 4, 5, 6, 7, 8, 9, 10]

Generate {.col-span-2}

>>>list(filter(lambdax : x%2==1, range(1, 20)))
[1, 3, 5, 7, 9, 11, 13, 15, 17, 19]
>>> [x**2forxinrange (1, 11) ifx%2==1]
[1, 9, 25, 49, 81]
>>> [xforxin [3, 4, 5, 6, 7] ifx>5]
[6, 7]
>>>list(filter(lambdax: x>5, [3, 4, 5, 6, 7]))
[6, 7]

Append

>>>li= []
>>>li.append(1)
>>>li
[1]
>>>li.append(2)
>>>li
[1, 2]
>>>li.append(4)
>>>li
[1, 2, 4]
>>>li.append(3)
>>>li
[1, 2, 4, 3]

List Slicing {.col-span-2 .row-span-3}

Syntax of list slicing:

a_list[start:end]
a_list[start:end:step]

Slicing

>>>a= ['spam', 'egg', 'bacon', 'tomato', 'ham', 'lobster']
>>>a[2:5]
['bacon', 'tomato', 'ham']
>>>a[-5:-2]
['egg', 'bacon', 'tomato']
>>>a[1:4]
['egg', 'bacon', 'tomato']

Omitting index

>>>a[:4]
['spam', 'egg', 'bacon', 'tomato']
>>>a[0:4]
['spam', 'egg', 'bacon', 'tomato']
>>>a[2:]
['bacon', 'tomato', 'ham', 'lobster']
>>>a[2:len(a)]
['bacon', 'tomato', 'ham', 'lobster']
>>>a
['spam', 'egg', 'bacon', 'tomato', 'ham', 'lobster']
>>>a[:]
['spam', 'egg', 'bacon', 'tomato', 'ham', 'lobster']

With a stride

['spam', 'egg', 'bacon', 'tomato', 'ham', 'lobster']
>>>a[0:6:2]
['spam', 'bacon', 'ham']
>>>a[1:6:2]
['egg', 'tomato', 'lobster']
>>>a[6:0:-2]
['lobster', 'tomato', 'egg']
>>>a
['spam', 'egg', 'bacon', 'tomato', 'ham', 'lobster']
>>>a[::-1]
['lobster', 'ham', 'tomato', 'bacon', 'egg', 'spam']

Remove

>>>li= ['bread', 'butter', 'milk']
>>>li.pop()
'milk'>>>li
['bread', 'butter']
>>>delli[0]
>>>li
['butter']

Access

>>>li= ['a', 'b', 'c', 'd']
>>>li[0]
'a'>>>li[-1]
'd'>>>li[4]
Traceback (mostrecentcalllast):
File"<stdin>", line1, in<module>IndexError: listindexoutofrange

Concatenating {.row-span-2}

>>>odd= [1, 3, 5]
>>>odd.extend([9, 11, 13])
>>>odd
[1, 3, 5, 9, 11, 13]
>>>odd= [1, 3, 5]
>>>odd+ [9, 11, 13]
[1, 3, 5, 9, 11, 13]

Sort & Reverse {.row-span-2}

>>>li= [3, 1, 3, 2, 5]
>>>li.sort()
>>>li
[1, 2, 3, 3, 5]
>>>li.reverse()
>>>li
[5, 3, 3, 2, 1]

Count

>>>li= [3, 1, 3, 2, 5]
>>>li.count(3)
2

Repeating

>>>li= ["re"] *3>>>li
['re', 're', 're']

Python Flow control {.cols-3}

Basic

a=5ifa>10:
print("a is totally bigger than 10.")
elifa<10:
print("a is smaller than 10.")
else:
print("a is indeed 10.")

One line

>>>a=330>>>b=200>>>r="a"ifa>belse"b">>>print(r)
a

else if

value=Trueifnotvalue:
print("Value is False")
elifvalueisNone:
print("Value is None")
else:
print("Value is True")

Python Loops {.cols-3}

Basic

primes= [2, 3, 5, 7]
forprimeinprimes:
print(prime)

With index

animals= ["dog", "cat", "mouse"]
fori, valueinenumerate(animals):
print(i, value)

While

x=0whilex<4:
print(x)
x+=1# Shorthand for x = x + 1

Break

x=0forindexinrange(10):
x=index*10ifindex==5:
breakprint(x)

Continue

forindexinrange(3, 8): x=index*10ifindex==5:
continueprint(x)

Range

foriinrange(4):
print(i) # Prints: 0 1 2 3foriinrange(4, 8):
print(i) # Prints: 4 5 6 7foriinrange(4, 10, 2):
print(i) # Prints: 4 6 8

With zip()

name= ['Pete', 'John', 'Elizabeth']
age= [6, 23, 44]
forn, ainzip(name, age):
print('%s is %d years old'%(n, a))

List Comprehension {.col-span-2}

result= [x**2forxinrange(10) ifx%2==0]
print(result)
# [0, 4, 16, 36, 64]

Python Functions {.cols-3}

Basic

defhello_world(): print('Hello, World!')

Return

defadd(x, y):
print("x is %s, y is %s"%(x, y))
returnx+yadd(5, 6) # => 11

Positional arguments

defvarargs(*args):
returnargsvarargs(1, 2, 3) # => (1, 2, 3)

Keyword arguments

defkeyword_args(**kwargs):
returnkwargs# => {"big": "foot", "loch": "ness"}keyword_args(big="foot", loch="ness")

Returning multiple

defswap(x, y):
returny, xx=1y=2x, y=swap(x, y) # => x = 2, y = 1

Default Value

defadd(x, y=10):
returnx+yadd(5) # => 15add(5, 20) # => 25

Anonymous functions

# => True
(lambdax: x>2)(3)
# => 5
(lambdax, y: x**2+y**2)(2, 1)

Python Modules {.cols-3}

Import modules

importmathprint(math.sqrt(16)) # => 4.0

From a module

frommathimportceil, floorprint(ceil(3.7)) # => 4.0print(floor(3.7)) # => 3.0

Import all

frommathimport*

Shorten module

importmathasm# => Truemath.sqrt(16) ==m.sqrt(16)

Functions and attributes

importmathdir(math)

Python File Handling {.cols-3}

Read file

Line by line

withopen("myfile.txt") asfile:
forlineinfile:
print(line)

With line number

input=open('myfile.txt', 'r')
fori,lineinenumerate(input, start=1):
print("Number %s: %s"% (i, line))

String

Write a string

contents= {"aa": 12, "bb": 21}
withopen("myfile1.txt", "w+") asfile:
file.write(str(contents))

Read a string

withopen('myfile1.txt', "r+") asfile:
contents=file.read()
print(contents)

Object

Write an object

contents= {"aa": 12, "bb": 21}
withopen("myfile2.txt", "w+") asfile:
file.write(json.dumps(contents))

Read an object

withopen('myfile2.txt', "r+") asfile:
contents=json.load(file)
print(contents)

Delete a File

importosos.remove("myfile.txt")

Check and Delete

importosifos.path.exists("myfile.txt"):
os.remove("myfile.txt")
else:
print("The file does not exist")

Delete Folder

importosos.rmdir("myfolder")

Python Classes & Inheritance {.cols-3}

Defining

classMyNewClass:
'''This is a docstring.'''pass# Class Instantiationmy=MyNewClass()

Constructors

classAnimal:
def__init__(self, voice):
self.voice=voicecat=Animal('Meow')
print(cat.voice) # => Meowdog=Animal('Woof') print(dog.voice) # => Woof

Method

classDog:
# Method of the classdefbark(self):
print("Ham-Ham")
charlie=Dog()
charlie.bark() # => "Ham-Ham"

Class Variables {.row-span-2}

classmy_class:
class_variable="A class variable!"x=my_class()
y=my_class()
# => A class variable!print(x.class_variable)
# => A class variable!print(y.class_variable)

Super() Function {.row-span-2}

classParentClass:
defprint_test(self):
print("Parent Method")
classChildClass(ParentClass):
defprint_test(self):
print("Child Method")
# Calls the parent's print_test()super().print_test() 

---

>>>child_instance=ChildClass()
>>>child_instance.print_test()
ChildMethodParentMethod

repr() method

classEmployee:
def__init__(self, name):
self.name=namedef__repr__(self):
returnself.namejohn=Employee('John')
print(john) # => John

User-defined exceptions

classCustomError(Exception):
pass

Polymorphism

classParentClass:
defprint_self(self):
print('A')
classChildClass(ParentClass):
defprint_self(self):
print('B')
obj_A=ParentClass()
obj_B=ChildClass()
obj_A.print_self() # => Aobj_B.print_self() # => B

Overriding

classParentClass:
defprint_self(self):
print("Parent")
classChildClass(ParentClass):
defprint_self(self):
print("Child")
child_instance=ChildClass()
child_instance.print_self() # => Child

Inheritance

classAnimal: def__init__(self, name, legs):
self.name=nameself.legs=legsclassDog(Animal):
defsound(self):
print("Woof!")
Yoki=Dog("Yoki", 4)
print(Yoki.name) # => YOKIprint(Yoki.legs) # => 4Yoki.sound() # => Woof!

Miscellaneous {.cols-3}

Comments

# This is a single line comments.

""" Multiline strings can be written using three "s, and are often used as documentation."""

''' Multiline strings can be written using three 's, and are often used as documentation.'''

Generators

defdouble_numbers(iterable):
foriiniterable:
yieldi+i

Generators help you make lazy code.

Generator to list

values= (-xforxin [1,2,3,4,5])
gen_to_list=list(values)
# => [-1, -2, -3, -4, -5]print(gen_to_list)

Handle exceptions {.col-span-3}

try:
# Use "raise" to raise an errorraiseIndexError("This is an index error")
exceptIndexErrorase:
pass# Pass is just a no-op. Usually you would do recovery here.except (TypeError, NameError):
pass# Multiple exceptions can be handled together, if required.else: # Optional clause to the try/except block. Must follow all except blocksprint("All good!") # Runs only if the code in try raises no exceptionsfinally: # Execute under all circumstancesprint("We can clean up resources here")