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Conditional expression with branches of type None and T should be Optional[T], not object #1820

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Conditional expression with branches of type None and T should be Optional[T], not object#1820
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topic-strict-optional

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@gvanrossum
gvanrossum
opened on Jul 7, 2016

Consider:

def f(flag; bool) -> Optional[int]:
  a = (None if flag else 0)
  return a

When run with --strict-optional I get the error

error: Incompatible return value type (got "object", expected "Optional[int]")

It looks like the unification code is not understanding what's going on and falling back on object.

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  1. gvanrossum
    added
    topic-strict-optional
    on Jul 7, 2016

  2. ddfisher commented on Jul 21, 2016

    @ddfisher
    ddfisher
    on Jul 21, 2016
    Collaborator

    Related to #1863. This is due to mypy's reluctance to make Union types.

  3. ddfisher
    added this to the milestone on Jul 21, 2016

  4. gnprice commented on Aug 1, 2016

    @gnprice
    gnprice
    on Aug 1, 2016
    Collaborator

    As discussed under #1863, we should go ahead and become willing to make Union types when one of the types to be combined is None. We might go back later and become less reluctant to make Union types more broadly, but there's more detailed discussion to be had there (see #1863.)

  5. gvanrossum
    closed this as completedin #1999on Aug 8, 2016
  6. gvanrossum
    added a commit that references this issue on Aug 8, 2016

    Make join(None, T) produce Optional[T] in strict-optional (#1999)

    d83aece
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