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pgjones opened this issue Apr 18, 2017 · 0 comments
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"Invalid type" error for Optional[int] alias #3191

pgjones opened this issue Apr 18, 2017 · 0 comments
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bug mypy got something wrong priority-1-normal

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pgjones commented Apr 18, 2017

The following snippet fails with 5: error: Invalid type "T" with mypy 0.501 on Python 3.5.2.

from typing import List, Optional

T = Optional[int]

Ts = []  # type: List[T]

This is similar to #1637 and was originally discussed there.
@ilevkivskyi ilevkivskyi added bug mypy got something wrong priority-1-normal labels Apr 18, 2017
@ilevkivskyi ilevkivskyi self-assigned this Apr 18, 2017
@ilevkivskyi ilevkivskyi mentioned this issue Jun 11, 2017
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@ilevkivskyi ilevkivskyi mentioned this issue Jun 23, 2017
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JukkaL pushed a commit that referenced this issue Jul 7, 2017
@ilevkivskyi @JukkaL
Simplify and tighten type aliases (#3524)
70d5f45 
Fixes #2887. Fixes #3191.

In addition this prohibits reassigning aliases. Previously something like 
this was allowed:

```
if random():
    Alias = Sequence[int]
else:
    Alias = Sequence[float]

def fun(arg: Alias) -> None:
    ...
```

Now this will generate an error: 

    Cannot assign multiple types to name "Alias" without an explicit "Type[...]" 
    annotation. 

See #3494 for background.

Finally, this simplifies the logic in semanal.py, so that most processing of type aliases 
happens in one function.
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