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
  • Static, dynamic, and gradual typing
  • Fully static and gradual types
  • Subtype, supertype, and type equivalence
  • Nominal and structural types
  • Materialization
  • Consistency
  • The assignable-to (or consistent subtyping) relation
  • Summary of type relations
  • Attributes and methods
  • Union types
  • References
• Type annotations
  • The meaning of annotations
  • Type and annotation expressions
  • String annotations
  • Annotating generator functions and coroutines
  • Annotating instance and class methods
• Type forms
  • TypeForm
  • Implicit TypeForm evaluation
  • Valid type expressions
  • Explicit TypeForm evaluation
  • Assignability
• 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 assignability
  • 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
  • Assignability 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
  • Terminology
  • Positional-only parameters
  • Default argument values
  • Annotating *args and **kwargs
  • Unpack for keyword arguments
  • Callable
  • Callback protocols
  • Assignability rules for callables
• 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
  • Metaclass Constructors
  • The type Constructor
  • Class Statements
  • The __init_subclass__() Method
• Overloads
  • Overload definitions
  • Invalid overload definitions
  • Implementation consistency
  • Overload call evaluation
  • Argument type expansion
• Exceptions
  • Context Managers
• Dataclasses
  • The dataclass_transform decorator
• Typed dictionaries
  • Syntax
  • Using TypedDict Types
  • Inheritance
  • Subtyping and assignability
  • Supported and Unsupported Operations
  • Backwards Compatibility
• Tuples
  • Tuple Type Form
  • Unpacked Tuple Form
  • Type Compatibility Rules
• Named Tuples
  • Defining Named Tuples
  • Named Tuple Usage
  • Assignability
• Enumerations
  • Enum Definition
  • Enum Behaviors
  • Defining Members
  • Member Names
  • Member Values
  • Enum Literal Expansion
• 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
  • @disjoint_base
• Distributing type information
  • Stub files
  • Type information in libraries
  • Library interface (public and private symbols)
  • Import Conventions
• Historical and deprecated features
  • Type comments
  • Type comments on function definitions
  • Positional-only parameters
  • Generics in standard collections
  • Union and Optional
  • Unpack
• Glossary