Welcome to Mypy documentation!

Mypy is a static type checker for Python 3 and Python 2.7.

First steps

• Introduction
• Getting started
  • Installing mypy
  • Running mypy
  • Function signatures
  • The typing module
  • Library stubs and typeshed
  • Next steps
• Using mypy with an existing codebase
  • Start small
  • Mypy runner script
  • Continuous Integration
  • Annotate widely imported modules
  • Write annotations as you go
  • Automate annotation of legacy code
  • Speed up mypy runs
  • Introduce stricter options

Cheat sheets

• Type hints cheat sheet (Python 3)
  • Variables
  • Built-in types
  • Functions
  • When you’re puzzled or when things are complicated
  • Standard “duck types”
  • Classes
  • Coroutines and asyncio
  • Miscellaneous
• Type hints cheat sheet (Python 2)
  • Built-in types
  • Functions
  • When you’re puzzled or when things are complicated
  • Standard “duck types”
  • Classes
  • Miscellaneous

Type system reference

• Built-in types
• Type inference and type annotations
  • Type inference
  • Explicit types for variables
  • Explicit types for collections
  • Compatibility of container types
  • Context in type inference
  • Declaring multiple variable types at a time
  • Starred expressions
• Kinds of types
  • Class types
  • The Any type
  • Tuple types
  • Callable types (and lambdas)
  • Union types
  • Optional types and the None type
  • Disabling strict optional checking
  • Class name forward references
  • Type aliases
  • Named tuples
  • The type of class objects
  • Text and AnyStr
  • Generators
• Class basics
  • Instance and class attributes
  • Overriding statically typed methods
  • Abstract base classes and multiple inheritance
• Protocols and structural subtyping
  • Predefined protocols
  • Simple user-defined protocols
  • Defining subprotocols and subclassing protocols
  • Recursive protocols
  • Using isinstance() with protocols
• Type checking Python 2 code
  • Multi-line Python 2 function annotations
  • Additional notes
• Dynamically typed code
  • Operations on Any values
  • Any vs. object
• Casts and type assertions
• Duck type compatibility
• Stub files
  • Creating a stub
  • Stub file syntax
• Generics
  • Defining generic classes
  • Generic class internals
  • Defining sub-classes of generic classes
  • Generic functions
  • Generic methods and generic self
  • Variance of generic types
  • Type variables with value restriction
  • Type variables with upper bounds
  • Declaring decorators
  • Generic protocols
  • Generic type aliases
• More types
  • The NoReturn type
  • NewTypes
  • Function overloading
  • Typing async/await
  • TypedDict
• Metaclasses
  • Defining a metaclass
  • Metaclass usage example
  • Gotchas and limitations of metaclass support

Configuring and running mypy

• Running mypy and managing imports
  • Specifying code to be checked
  • Reading a list of files from a file
  • How mypy handles imports
  • Mapping file paths to modules
  • How imports are found
• The mypy command line
  • Specifying what to type check
  • Config file
  • Import discovery
  • Platform configuration
  • Disallow Any
  • Untyped definitions and calls
  • None and Optional handling
  • Configuring warnings
  • Miscellaneous strictness flags
  • Configuring error messages
  • Incremental mode
  • Advanced flags
  • Report generation
  • Miscellaneous
• The mypy configuration file
  • Global flags
  • Per-module flags
  • Examples
• Mypy daemon (mypy server)
  • Basic usage
  • Additional features
  • Limitations
• Using installed packages
  • Using PEP 561 compatible packages with mypy
  • Making PEP 561 compatible packages
• Extending and integrating mypy
  • Integrating mypy into another Python application
  • Extending mypy using plugins

Miscellaneous

• Common issues and solutions
  • Can’t install mypy using pip
  • No errors reported for obviously wrong code
  • Spurious errors and locally silencing the checker
  • Unexpected errors about ‘None’ and/or ‘Optional’ types
  • Mypy runs are slow
  • Types of empty collections
  • Redefinitions with incompatible types
  • Invariance vs covariance
  • Declaring a supertype as variable type
  • Complex type tests
  • Python version and system platform checks
  • Displaying the type of an expression
  • Import cycles
  • Silencing linters
  • Covariant subtyping of mutable protocol members is rejected
  • Dealing with conflicting names
  • I need a mypy bug fix that hasn’t been released yet
• Supported Python features
  • Runtime definition of methods and functions
• New features in Python 3.6
  • Syntax for variable annotations (PEP 526)
  • Asynchronous generators (PEP 525) and comprehensions (PEP 530)
  • New named tuple syntax
• Additional features
  • The attrs package
  • Using a remote cache to speed up mypy runs
  • Extended Callable types
• Frequently Asked Questions
  • Why have both dynamic and static typing?
  • Would my project benefit from static typing?
  • Can I use mypy to type check my existing Python code?
  • Will static typing make my programs run faster?
  • How do I type check my Python 2 code?
  • Is mypy free?
  • Can I use duck typing with mypy?
  • I like Python and I have no need for static typing
  • How are mypy programs different from normal Python?
  • How is mypy different from Cython?
  • Mypy is a cool project. Can I help?
• Revision history

Indices and tables

• Index
• Search Page