mypy doesn't work nicely with lambdas returning named tuples

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#5,069 6 comments 0 reactions 0 assignees View on GitHub

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Assessment

Difficulty
4/5
Estimated time
3-5 days
Newbie friendliness
35/100
Issue type
Bug
Clarity
Clearly specified
Activity status
Stale
Tech stack
python
Domain
compilers, devtools

Research direction

Start by running the minimal repro in a.py with mypy --strict and compare the reported error with the expected behavior for lambda bidirectional type inference described in the issue. Trace how the lambda returning NamedTuple A is inferred, then add or update a focused regression test showing that greet is typed without an unknown-function error.

Written by the indexing model from the issue text.

Description

documentation topic-usability

Note: if you are reporting a wrong signature of a function or a class in
the standard library, then the typeshed tracker is better suited
for this report: https://github.com/python/typeshed/issues

Please provide more information to help us understand the issue:

  • Are you reporting a bug, or opening a feature request? bug
  • Please insert below the code you are checking with mypy,
    or a mock-up repro if the source is private. We would appreciate
    if you try to simplify your case to a minimal repro.
from typing import NamedTuple


class A(NamedTuple):
    '''
    value class combining origin coordinate with list of destination coordinates
    '''
    a: str


def foo() -> None:
    A("david")
    greet = lambda a: A("hi " + a)
    greet('moshe')

Lambdas are also supported. The lambda argument and return value types cannot be given explicitly; they are always inferred based on context using bidirectional type inference:

  • What are the versions of mypy and Python you are using?
    the latest and greatest: mypy 0.600 and python 3.6.5
    Do you see the same issue after installing mypy from Git master? Didn't try
  • What are the mypy flags you are using? (For example --strict-optional)
    --strict
Dominant language
Python
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