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PEP 484: Do not require type checkers to treat a None default specially #689

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Jul 10, 2018
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PEP 484: Do not require type checkers to treat a None default specially #689

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Do not require type checkers to treat a None default specially
JelleZijlstra Jul 1, 2018
849f9b0
Be more explicit that we're moving away from implicit optional
JelleZijlstra Jul 10, 2018
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8 changes: 5 additions & 3 deletions pep-0484.txt
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Expand Up @@ -984,15 +984,17 @@ for example, the above is equivalent to::

def handle_employee(e: Optional[Employee]) -> None: ...

An optional type is also automatically assumed when the default value is
``None``, for example::
A past version of this PEP allowed type checkers to assume an optional
type when the default value is ``None``, as in this code::

def handle_employee(e: Employee = None): ...

This is equivalent to::
This would have been treated as equivalent to::

def handle_employee(e: Optional[Employee] = None) -> None: ...

This is no longer the recommended behavior. Type checkers should move
towards requiring the optional type to be made explicit.

Support for singleton types in unions
-------------------------------------
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