Specification for the Python type system

Contents:

• The Python Type System
  • Purpose
  • Non-goals
  • Interpretation
• Meta-topics
  • About this specification
  • Changing the specification
• Type system concepts
  • Union types
• Type annotations
  • The meaning of annotations
  • Type and annotation expressions
  • String annotations
  • Annotating generator functions and coroutines
  • Annotating instance and class methods
• Special types in annotations
  • Any
  • None
  • NoReturn
  • Never
  • Special cases for float and complex
  • type[]
• Generics
  • Introduction
  • User-defined generic types
  • Scoping rules for type variables
  • Instantiating generic classes and type erasure
  • Arbitrary generic types as base classes
  • Abstract generic types
  • Type variables with an upper bound
  • Variance
  • ParamSpec
  • TypeVarTuple
  • Defaults for Type Parameters
  • Self
  • Variance Inference
• Type qualifiers
  • @final
  • Final
  • Annotated
• Class type compatibility
  • ClassVar
  • @override
• Type aliases
  • TypeAlias
  • type statement
  • NewType
• Literals
  • Literal
  • LiteralString
• Protocols
  • Terminology
  • Defining a protocol
  • Protocol members
  • Explicitly declaring implementation
  • Merging and extending protocols
  • Generic protocols
  • Recursive protocols
  • Self-types in protocols
  • Subtyping relationships with other types
  • Unions and intersections of protocols
  • type[] and class objects vs protocols
  • NewType() and type aliases
  • Modules as implementations of protocols
  • @runtime_checkable decorator and narrowing types by isinstance()
• Callables
  • Argument defaults
  • Annotating *args and **kwargs
  • Unpack for keyword arguments
  • Callable
  • Callback protocols
• Constructors
  • Constructor Calls
  • Metaclass __call__ Method
  • __new__ Method
  • __init__ Method
  • Classes Without __new__ and __init__ Methods
  • Constructor Calls for type[T]
  • Specialization During Construction
  • Consistency of __new__ and __init__
  • Converting a Constructor to Callable
• @overload
• Exceptions
  • Context Managers
• Dataclasses
  • The dataclass_transform decorator
• Typed dictionaries
  • TypedDict
  • Required and NotRequired
  • Read-only Items
• Tuples
  • Tuple Type Form
  • Unpacked Tuple Form
  • Type Compatibility Rules
• Named Tuples
  • Defining Named Tuples
  • Named Tuple Usage
  • Type Compatibility Rules
• Type narrowing
  • TypeGuard
  • TypeIs
• Type checker directives
  • assert_type()
  • reveal_type()
  • # type: ignore comments
  • cast()
  • TYPE_CHECKING
  • @no_type_check
  • Version and platform checking
  • @deprecated
• Distributing type information
  • Stub files
  • Type information in libraries
• Historical and deprecated features
  • Type comments
  • Type comments on function definitions
  • Positional-only parameters
  • Generics in standard collections
  • Union and Optional
  • Unpack
• Glossary