Python — Syntax

Indentation defines structure

Most languages use curly braces {} to group code into blocks. Python uses indentation — whitespace at the start of a line:

if age >= 18:
    print("Adult")       # indented block
    print("You can vote")
else:
    print("Minor")       # indented block

The standard is 4 spaces per indent level. Using tabs or mixing spaces and tabs causes TabError. Your editor should be configured to insert 4 spaces when you press Tab.

No semicolons

Python doesn't use semicolons to end statements. Each line is its own statement:

name = "Ada"
age = 25
print(name)

You can put multiple statements on one line with semicolons, but don't:

# Legal but ugly
name = "Ada"; age = 25; print(name)

One statement per line is the Python way.

Line continuation

Python statements end at the line break. For long expressions, wrap in parentheses:

total = (price1 + price2 + price3
         + tax + shipping)

Or use a backslash (less preferred):

total = price1 + price2 + price3 \
        + tax + shipping

Parentheses are cleaner — they work for expressions, function calls, and list comprehensions.

Comments

# This is a comment — the interpreter ignores it

x = 5  # inline comment also works

Python has no multi-line comment syntax. For multi-line explanations, use string literals:

"""
This module provides utility functions
for data processing and validation.
"""

# Regular code continues here
def process(data):
    pass

Triple-quoted strings at the top of a file serve as module documentation (docstrings). They're not technically comments, but they function as documentation.

Naming conventions

Python follows PEP 8 — the community style guide:

Element	Convention	Example
Variables	snake_case	student_count
Functions	snake_case	get_average
Classes	PascalCase	StudentRecord
Constants	UPPER_SNAKE_CASE	MAX_USERS
Modules	snake_case	my_module.py

# Good
def calculate_average(scores):
    total = sum(scores)
    return total / len(scores)

# Bad
def CalculateAverage(Scores):
    Total = sum(Scores)
    return Total / len(Scores)

Python doesn't enforce conventions, but the community strongly follows PEP 8. Most editors have PEP 8 checking built in.

Dynamic typing

Python infers types at runtime — no type declarations needed:

x = 5           # int
x = "hello"     # now a str — no error
x = [1, 2, 3]   # now a list

Variables can hold any type at any time. This flexibility is powerful but can hide bugs. For larger projects, use type hints:

def greet(name: str) -> str:
    return f"Hello, {name}!"

Type hints don't enforce types at runtime — they help editors and tools like mypy catch bugs before they happen.

Imports

# Import the whole module
import math
print(math.sqrt(16))  # 4.0

# Import specific items
from math import sqrt, pi
print(sqrt(16))  # 4.0
print(pi)        # 3.141592653589793

# Import with an alias
import numpy as np
arr = np.array([1, 2, 3])

# Import everything (avoid this)
from math import *

Import at the top of the file. Standard library imports come first, then third-party, then local — separated by blank lines.

The Zen of Python

Type import this in a Python interpreter and you'll see the language's philosophy:

Beautiful is better than ugly.
Explicit is better than implicit.
Simple is better than complex.
Complex is better than complicated.
Readability counts.

These principles guide Python's design. Code that follows them is considered "Pythonic."

Whitespace and readability

Python ignores extra spaces in expressions:

x   =   5
y =     10 +     20

But the community enforces clean formatting:

x = 5
y = 10 + 20

Use black or ruff format to auto-format your code. No debates about formatting — the tool decides.

Executable vs interactive

Python runs in two modes:

# Interactive mode — type a line, see the result
>>> print("hello")
hello

# Script mode — save a .py file and run it
$ python my_script.py

Interactive mode is great for experiments. Script mode is for real programs.

Errors are helpful

Python's error messages tell you what went wrong and where:

def greet(name)
    print(f"Hello, {name}")

Output:

  File "main.py", line 1
    def greet(name)
                  ^
SyntaxError: expected ':'

The ^ points to the exact location. Python errors include file name, line number, and a description. Read them carefully — they usually tell you exactly how to fix the issue.

Mini Practice

1. Write a program that uses proper indentation with an if-else statement
2. Add a multi-line comment at the top of a file describing what it does
3. Import random and print a random number between 1 and 100
4. Write a function with a type hint that takes a string and returns its length
5. Run import this in a Python shell and pick your favorite Zen principle

Next: commenting your code →