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None gets dropped in ternary statement with lists #2258

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ddfisher opened this issue Oct 14, 2016 · 2 comments
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None gets dropped in ternary statement with lists #2258

ddfisher opened this issue Oct 14, 2016 · 2 comments
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python3 -m mypy --strict-optional -c 'reveal_type([None] if bool() else [1])' <string>:1: error: Revealed type is 'builtins.list[builtins.int]'
@ddfisher ddfisher self-assigned this Oct 14, 2016
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refi64 commented Oct 14, 2016 •
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This is basically just a duplicate of #2246. is_valid_inferred_type is False for [None] since it's technically invalid without a type annotation, so visit_conditional_expr just discards the type of that branch. Pretty sure fixing #2246 would also fix this.

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Nice catch! It looks that way to me too. This makes me feel like our decision on #2246 is definitely right.

@ddfisher ddfisher mentioned this issue Oct 20, 2016
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gvanrossum pushed a commit that referenced this issue Oct 22, 2016
@ddfisher @gvanrossum
Don't turn collections containing None into partial types (#2289)
ad62a08 
This means mypy will no longer give Need type annotation for variable errors for assignments with values of type Generator[str, None, None] or List[None] or similar. However, lists, sets, and dicts that are initialized with None values that were previously inferred may now need type annotations. I.e. constructs that look like:

x = [None]
x.append(0)

will now be type errors unless they have explicit annotations.

Supplants #2225. Fixes #2246. Fixes #2195. Fixes #2258.
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