PEP 8 and Style

PEP 8 is Python’s official style guide. Following it makes code readable and consistent across the entire Python ecosystem.

Naming Conventions

Element	Convention	Example
Variable, function	snake_case	user_name, calculate_total()
Class	PascalCase	UserAccount, HTTPRequest
Constant	UPPER_SNAKE_CASE	MAX_RETRIES, API_KEY
Module	lowercase or snake_case	utils.py, data_loader.py
Private/internal (convention only)	leading underscore	_internal_helper()
Name mangled (see Encapsulation-and-Properties)	leading double underscore	__secret

Indentation and Line Length

# 4 spaces per indentation level, never tabs
def func():
    if True:
        do_something()
 
# Max line length: 79 chars (strict PEP 8) or 88 (Black formatter default), team-dependent

Whitespace Rules

# Good
x = 1
y = x + 1
func(a, b, c)
d = {"key": "value"}
 
# Avoid
x=1
y = x+1
func( a,b,c )
d = { "key" : "value" }

# Two blank lines before top-level function/class definitions
def first_function():
    pass
 
 
def second_function():
    pass
 
 
class MyClass:
    # One blank line between methods within a class
    def method_one(self):
        pass
 
    def method_two(self):
        pass

Imports

# Standard library first, then third-party, then local application imports, each group separated by a blank line
import os
import sys
 
import requests
import pandas as pd
 
from my_package import my_module

One import per line for import, grouping allowed for from

import os
import sys              # preferred over: import os, sys
 
from typing import List, Dict, Optional     # fine to group in a single from-import

Comparisons

# Good
if x is None:
    ...
if not items:
    ...
 
# Avoid
if x == None:
    ...
if len(items) == 0:
    ...

String Quotes

PEP 8 doesn’t mandate single vs double quotes, just consistency. Most modern tooling (Black) defaults to double quotes.

Docstring Conventions

def calculate_total(items, tax_rate=0.0):
    """Calculate the total cost including tax.
 
    Args:
        items: List of item prices.
        tax_rate: Tax rate as a decimal (e.g. 0.08 for 8%).
 
    Returns:
        The total cost as a float.
    """
    subtotal = sum(items)
    return subtotal * (1 + tax_rate)

Automated Tooling (Do This Instead of Manual Checking)

pip install black flake8 isort ruff
 
black my_file.py         # auto-formats code to a consistent style
isort my_file.py            # auto-sorts and groups imports
flake8 my_file.py              # lints for PEP 8 violations and common errors
ruff check my_file.py             # fast, modern linter, increasingly replacing flake8

Don't manually enforce style, automate it

Running black on save (most editors support this) means you never have to think about spacing or line-wrapping decisions again. Reserve mental effort for logic, not formatting.

The Zen of Python

import this

Prints PEP 20, a short list of guiding principles (“Beautiful is better than ugly”, “Explicit is better than implicit”, “Simple is better than complex”, “Readability counts”). Worth reading once, genuinely.