01_python_basics

01 · Python Basics

Introduction

Python is a high-level, interpreted, dynamically-typed programming language known for readable syntax and a "batteries included" standard library. This chapter covers what Python is, how it runs, and how to write and execute your first programs.

Theory

When you run a .py file, CPython (the reference implementation) compiles your source into bytecode, then executes that bytecode on the Python Virtual Machine (PVM). This is why Python is often called "interpreted" even though a compilation step happens internally.

flowchart LR
A[source.py] --> B[Compiler]
B --> C[Bytecode .pyc]
C --> D[Python Virtual Machine]
D --> E[Output]

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Key characteristics:

• Dynamically typed — variable types are checked at runtime, not compile time.
• Interpreted — no separate manual compilation step for the developer.
• Indentation-based — blocks are defined by whitespace, not braces.
• Multi-paradigm — supports procedural, object-oriented, and functional styles.

Syntax

# This is a commentprint("Hello, World!") # function call# Statements end at the newline, no semicolon requiredx=5y=10print(x+y)

Examples

See src/01_python_basics/hello_world.py and src/01_python_basics/comments.py for runnable code.

Code Explanation

• print() is a built-in function that writes to standard output.
• Python statements are executed top-to-bottom; there's no main() requirement, though larger programs conventionally use an if __name__ == "__main__": guard (introduced properly in Chapter 07: Modules).
• The # character starts a comment; everything after it on that line is ignored by the interpreter.

Best Practices

• Save files with the .py extension and use snake_case filenames.
• Use python3 explicitly on systems where python may point to Python 2.
• Keep one statement per line — Python allows ;-separated statements on one line, but it hurts readability.
• Follow PEP 8 from day one; it becomes muscle memory faster than you'd expect.

Common Mistakes

Mistake	Why it's a problem	Fix
Mixing tabs and spaces	Causes TabError or silent misalignment	Configure your editor to insert spaces only
Forgetting print() parentheses (Python 2 habit)	print "x" is a SyntaxError in Python 3	Always use print(x)
Assuming python = Python 3	Some systems still alias python to Python 2	Use python3 or check python --version

Interview Questions

1. What's the difference between a compiled and an interpreted language, and where does Python sit?
2. What is CPython, and how does it relate to "Python"?
3. Why does Python use indentation instead of braces?

Exercises

1. Write a script that prints your name, your favorite language feature, and today's date (hardcoded is fine for now).
2. Modify hello_world.py to print three lines using three separate print() calls, then again using a single call with \n.
3. Add three different types of comments (line comment, inline comment, block comment) to a script.

Further Reading

• Python official tutorial
• PEP 8 Style Guide

Related Topics

• 02 · Variables
• 07 · Modules