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PEP 696: Update pep-0696.rst to make the explanations of Abstract and Motivation section clearer #4722

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PEP 696: Update pep-0696.rst to make the explanations of Abstract and Motivation section clearer #4722

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Update pep-0696.rst
hyperkai Dec 2, 2025
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hyperkai Dec 2, 2025
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22 changes: 18 additions & 4 deletions peps/pep-0696.rst
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Expand Up @@ -20,7 +20,9 @@ Abstract

This PEP introduces the concept of type defaults for type parameters,
including ``TypeVar``, ``ParamSpec``, and ``TypeVarTuple``,
which act as defaults for type parameters for which no type is specified.
which act as defaults for type parameters for which no type arguments
are specified explicitly with `'[]'`, or implicitly without `'[]'`
but with the type inference by an assigned value.

Default type argument support is available in some popular languages
such as C++, TypeScript, and Rust. A survey of type parameter syntax in
Expand All @@ -34,15 +36,27 @@ Motivation

::

T = TypeVar("T", default=int) # This means that if no type is specified T = int

T = TypeVar("T", default=int) # This means that T = int if no type argument is specified explicitly
# with `'[]'`, or implicitly without `'[]'` but with the type
# inference by an assigned value.
@dataclass
class Box(Generic[T]):
value: T | None = None

reveal_type(Box()) # type is Box[int]
# The type argument isn't specified.
reveal_type(Box()) # type is Box[int]

# The type argument is specified with '[]'.
reveal_type(Box[str]()) # type is Box[str]

# The type argument is specified with
# the type inference by an assigned value.
reveal_type(Box(value="Hello World!")) # type is Box[str]

# The type argument is specified with '[]' but not with the type
# inference by an assigned value because '[]' is prioritized.
reveal_type(Box[str](value="Hello World!")) # type is Box[str]

One place this `regularly comes
up <https://github.com/python/typing/issues/975>`__ is ``Generator``. I
propose changing the *stub definition* to something like::
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