Forward reference to a type alias results in an "Invalid type ..." error

mypy 0.4.5 and 0.4.4, CPython 3.5.1 and 3.5.2.

Code:

# code.py
def f(p: 'Alias') -> None:
    print(p)

Alias = int

Expected result:

% mypy code.py
%

Actual result:

% mypy code.py
code.py: note: In function "f":
code.py:1: error: Invalid type "code.Alias"
%

Hm, interesting. Looks like aliases can't be used in forward references. This is unfortunate, but shouldn't be hard to work around, right? I don't know yet what it would take to fix (both aliases and forward references are somewhat murky corners of mypy).

I think that type aliases also don't work reliably across different modules in import cycles.

As far as I can tell, it is currently not possible to represent the output of json.load/s with types due to this.

JSON = Union[None, str, int, float, bool, List['JSON'], Dict[str, 'JSON']]

Adding newtypes in various bits didn't help the situation, I'm still getting 'Invalid type'

Here's another example I ran into:

from typing import List

class A(object):
    b: 'B'  # E: Invalid type "recurse.B"

B = List[A]

I'm also seeing this occur with new Python 3.6-style NamedTuple type hint declarations.

import typing as typ

class EdgeDef(typ.NamedTuple):
    id: int
    stack_cmd: 'StackCommand' # ERROR: Invalid type "test.StackCommand"

# Other defs that depend on `EdgeDef`

StackCommand = typ.Tuple[bool, int]

I think we have enough examples at this point. :-)

@ThatsGobbles Is that the same issue though? This issue is with recursive type aliases, but your example doesn't actually have any recursion, just and out-of-order alias.

@kirbyfan64

@kirbyfan64

This issue is with recursive type aliases

The original example does not have any recursion. Moreover, I am quite sure that there is no big difference between recursive and non-recursive situations with classes. The problem is very simple: type aliases are not analysed during first pass, but solution is not easy and requires refactoring semantic analyser (see roadmap).

cough I...uhh... totally knew that... cough

Fix crashes and fails in forward references (#3952)

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Forward reference to a type alias results in an "Invalid type ..." error #2241

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Forward reference to a type alias results in an "Invalid type ..." error #2241

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gvanrossum commented on Oct 11, 2016

JukkaL commented on Oct 11, 2016

TRManderson commented on Dec 1, 2016

JelleZijlstra commented on May 23, 2017

linclelinkpart5 commented on May 30, 2017

gvanrossum commented on May 31, 2017

refi64 commented on May 31, 2017

ilevkivskyi commented on May 31, 2017

refi64 commented on May 31, 2017

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