Introduction 🌟

mypy is a popular static type checker for Python. It reads your code, analyzes your type hints, and warns you about type mismatches — long before your program runs.

1. Installing mypy 📦

pip install mypy

✔ Install globally or inside a virtual environment

2. Running mypy on a File ▶️

mypy script.py

mypy will analyze script.py and show errors if types don't match.

3. Basic Example — Detecting Type Errors 🚨

def add(x: int, y: int) -> int:
    return x + y

result = add("10", 20)  # ❌ wrong type
print(result)

error: Argument 1 to "add" has incompatible type "str"; expected "int"

✔ mypy catches the error even before running the program

4. Type Checking Variables 🧱

age: int = "twenty"  # ❌

error: Incompatible types in assignment (expression has type "str", variable has type "int")

5. Using --strict Mode 🔒

For production-grade projects, strict mode provides maximum safety.

mypy --strict app.py

6. Optional Types & mypy ⚠️

mypy warns you if you use a value that might be None.

from typing import Optional

def get_user() -> Optional[str]:
    return None

name = get_user()
print(name.upper())   # ❌ possible None

error: Item "None" of "Optional[str]" has no attribute "upper"

✔ Fix with a None check

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

7. Type Checking Collections 📦

nums: list[int] = ["a", "b"]   # ❌ wrong types

8. Mypy with Classes & Methods 🧩

class User:
    def __init__(self, name: str, age: int):
        self.name = name
        self.age = age

u = User("Sathish", "25")  # ❌

error: Argument 2 to "User" has incompatible type "str"; expected "int"

9. Using reveal_type() for Debugging 🔍

x = [1, 2, 3]
reveal_type(x)

note: Revealed type is "builtins.list[builtins.int]"

✔ Great for debugging type inference

10. Ignoring Specific Lines 🙈

value: int = "hello"  # type: ignore

✔ Useful when integrating legacy code

11. Type Aliases & mypy 🎭

UserId = int

def get_user(uid: UserId) -> str:
    return "User"

✔ Clearer type standards for large teams

12. mypy + Generics 🔧

from typing import TypeVar, Generic

T = TypeVar("T")

class Box(Generic[T]):
    def __init__(self, value: T):
        self.value = value

b = Box("hello")
print(b.value.upper())

✔ mypy checks generic types properly

13. mypy Configuration File (optional) ⚙️

[mypy]
python_version = 3.11
warn_unused_configs = True
ignore_missing_imports = True
strict = True

✔ Place this in project root to configure once

14. Common Real-World Uses 🌍

15. mypy + VSCode / PyCharm 🖥️

Both IDEs can run mypy automatically while typing.

Cheat Sheet 📘

Best Practices 💡

Conclusion 🎉

You now understand Mypy Static Checking clearly! Want the next topic? Try Protocols, Dataclasses, Pydantic, or Unit Testing. Just tell me! 😊