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Mypy: static typing for Python

Mypy is a static type checker for Python that helps you find bugs before you even run your code. Add type hints to your Python programs, and mypy will warn you when you use those types incorrectly.
Python is a dynamic language, so you’ll usually only see errors when you run your code. Mypy is a static checker, so it finds bugs without running them.
Here’s a simple example of mypy catching a bug: 
number = input("What is your favourite number?")
print("It is", number + 1)  # error: Unsupported operand types for + ("str" and "int")
Mypy spotted that input() returns a string, not a number, so adding 1 to it will fail at runtime.

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Key features

Gradual typing

Add type hints incrementally to your codebase. Mix typed and untyped code freely.

Powerful type system

Support for generics, callable types, tuple types, union types, structural subtyping, and more.

Non-invasive

Type hints are like comments - your code runs the same with or without them.

Fast incremental checking

Daemon mode provides sub-second incremental type checking for large codebases.

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How it works

Type hints don’t interfere with how your program runs. Think of them as enhanced comments: 
def greeting(name):
    return 'Hello ' + name

greeting(123)  # This will crash at runtime, but mypy won't catch it
You can always use the Python interpreter to run your code, even if mypy reports errors. Type checking is completely optional.

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Gradual typing

Mypy is designed with gradual typing in mind. You can:
  • Add type hints to your codebase slowly, file by file or function by function
  • Always fall back to dynamic typing when static typing isn’t convenient
  • Mix typed and untyped code in the same project
This makes it easy to adopt mypy incrementally, whether you’re:
  • Migrating an existing Python codebase to use static types
  • Prototyping new features dynamically, then adding types later
  • Working with third-party libraries that don’t have type hints

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Type system features

Mypy has a powerful and easy-to-use type system that supports:

Type inference

Automatically deduce types from context without requiring annotations everywhere.

Generics

Create reusable code that works with multiple types, like list[int] or dict[str, float].

Callable types

Express function signatures as types, enabling higher-order functions.

Union types

Allow values to be one of several types using int | str syntax.

Tuple types

Fixed-size tuples with per-element types like tuple[int, str, float].

Structural subtyping

Types are compatible based on their structure (protocols), not just inheritance.

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Why use mypy?

Using mypy makes your programs:
  • Easier to understand - Type hints serve as machine-checked documentation
  • Easier to debug - Catch type errors before runtime, not during production
  • Easier to maintain - Refactor with confidence, knowing the type checker has your back
With the --strict flag, you can achieve near-zero runtime type errors. You’ll basically never get a type-related error at runtime without a corresponding mypy error.

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Getting started

Installation

Install mypy using pip and verify your setup.

Quickstart

Type-check your first Python program in minutes.

Cheat sheet

Quick reference for common type hints and patterns.

Configuration

Configure mypy for your project’s needs.

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Compiled with mypyc

Mypy is compiled using mypyc, making it approximately 4x faster than interpreted Python. Mypyc uses Python type hints to compile Python modules to faster C extensions.
This means mypy can type-check large codebases quickly, often in under a second with incremental mode.

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Next steps

Ready to start using mypy? Check out the installation guide or jump straight to the quickstart.

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