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Add some info about get_proper_type() #7950

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Merged
merged 1 commit into from
Nov 14, 2019
Merged

Add some info about get_proper_type() #7950

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2 changes: 2 additions & 0 deletions misc/proper_plugin.py
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Original file line number Diff line number Diff line change
Expand Up @@ -42,6 +42,8 @@ def isinstance_proper_hook(ctx: FunctionContext) -> Type:
return ctx.default_return_type
ctx.api.fail('Never apply isinstance() to unexpanded types;'
' use mypy.types.get_proper_type() first', ctx.context)
ctx.api.note('If you pass on the original type' # type: ignore[attr-defined]
' after the check, always use its unexpanded version', ctx.context)
return ctx.default_return_type


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8 changes: 8 additions & 0 deletions mypy/types.py
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Expand Up @@ -1896,6 +1896,14 @@ def get_proper_type(typ: Type) -> ProperType: ...


def get_proper_type(typ: Optional[Type]) -> Optional[ProperType]:
"""Get the expansion of a type alias type.

If the type is already a proper type, this is a no-op. Use this function
wherever a decision is made on a call like e.g. 'if isinstance(typ, UnionType): ...',
because 'typ' in this case may be an alias to union. Note: if after making the decision
on the isinstance() call you pass on the original type (and not one of its components)
it is recommended to *always* pass on the unexpanded alias.
"""
if typ is None:
return None
while isinstance(typ, TypeAliasType):
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