mypy and flake8 Guide

A beginner-friendly guide to static analysis tools commonly used in Python projects.

This document explains: - mypy - flake8 - line too long - unused imports - indentation problems - naming conventions - common errors in 42 projects

These tools help improve: - code quality - readability - maintainability - type safety

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What is mypy?

mypy is a static type checker for Python.

It checks: - type hints - return types - Optional issues - invalid assignments - typing consistency

mypy analyzes code WITHOUT running it.

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Example

def add(a: int, b: int) -> int:
    return a + b

Correct typing: - parameters are integers - return value is integer

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Example Error

age: int = "hello"

mypy error:

Incompatible types in assignment

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Running mypy

mypy .

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Common mypy Errors

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Missing Return Statement

Example

def get_name() -> str:
    print("Sara")

Problem: - function says it returns str - but returns nothing

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Fix

def get_name() -> str:
    return "Sara"

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Optional Problems

Example

from typing import Optional

name: Optional[str] = None

print(name.upper())

Possible problem: - name may be None

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Fix

if name is not None:
    print(name.upper())

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Incompatible Types

Example

numbers: list[int] = ["hello"]

mypy error: - expected integer - received string

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What is flake8?

flake8 is a style and linting tool.

It checks: - formatting - style problems - unused imports - indentation - naming conventions

flake8 improves readability and consistency.

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Running flake8

flake8 .

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line too long

One of the most common flake8 errors.

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Example Error

E501 line too long

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Problem Example

print("This is an extremely long line that exceeds the maximum allowed line length in flake8")

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Why This Matters

Long lines: - reduce readability - are harder to review - look bad in terminals

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Fix

Split the line.

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Example Fix

print(
    "This is an extremely long line "
    "that was split correctly"
)

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unused imports

Very common error.

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Example Error

F401 'os' imported but unused

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Problem Example

import os

If os is never used: - flake8 complains

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Fix

Remove the import.

# removed unused import

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Why This Matters

Unused imports: - create clutter - confuse developers - slow large projects slightly

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indentation

Python depends on indentation.

Bad indentation causes: - syntax errors - readability problems

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Example Problem

def hello():
print("Hello")

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Correct Version

def hello():
    print("Hello")

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Recommended Indentation

PEP8 recommends: - 4 spaces

Avoid: - tabs mixed with spaces

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naming conventions

Python follows naming standards called: - PEP8 naming conventions

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Variables and Functions

Use:

snake_case

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Good Example

player_name = "Sara"

def calculate_score():
    pass

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Bad Example

PlayerName = "Sara"

def CalculateScore():
    pass

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Classes

Use:

PascalCase

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Good Example

class SpaceStation:
    pass

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Constants

Use:

UPPER_CASE

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Example

WINDOW_WIDTH = 1920

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Why Naming Matters

Good naming: - improves readability - explains intent - helps teamwork - simplifies debugging

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Common 42 flake8 Problems

Error	Meaning
E501	line too long
F401	unused import
E302	expected blank lines
E305	expected blank line after function
W293	blank line contains whitespace

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Example: Blank Line Error

Problem:

def hello():
    print("Hi")
def bye():
    print("Bye")

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Fix

def hello():
    print("Hi")


def bye():
    print("Bye")

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Strict mypy Mode

Many projects use:

mypy . --strict

This enables: - stricter checks - stronger typing rules

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Common Strict Errors

• missing annotations
• missing return types
• Any usage
• Optional problems

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Example

def add(a, b):
    return a + b

Strict mode complains because: - parameters are untyped

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Fix

def add(a: int, b: int) -> int:
    return a + b

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Good Workflow

Run both tools regularly:

flake8 .
mypy .

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Why Use Both?

flake8

Focuses on: - style - formatting - readability

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mypy

Focuses on: - typing - logic consistency - type safety

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Recommended 42 Workflow

1. Write code
2. Run program
3. Run flake8
4. Run mypy
5. Fix warnings immediately

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Best Practices

• Keep lines short
• Remove unused imports
• Use meaningful names
• Type all functions
• Avoid unnecessary Any
• Keep formatting consistent
• Run tools frequently

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Summary Table

Tool	Purpose
mypy	Static type checking
flake8	Style and linting
E501	Line too long
F401	Unused import
snake_case	Variable/function naming
PascalCase	Class naming

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Final Notes

mypy and flake8 are extremely important in professional Python development.

These tools help create: - cleaner code - safer projects - better teamwork - easier debugging - more maintainable applications

They are especially valuable in: - 42 projects - large Python applications - APIs - parsers - game engines - backend systems

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