Unable to specify a default value for a generic parameter

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#3,737 48 comments 121 reactions 0 assignees View on GitHub

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Assessment

Difficulty
5/5
Estimated time
Over a week
Newbie friendliness
35/100
Issue type
Feature
Clarity
Mostly clear
Activity status
Active
Tech stack
python
Domain
devtools

Research direction

The issue provides Python examples but names no implementation files or tests. Start by reproducing both examples and determine the intended typing behavior for generic defaults and Callable defaults; then locate the relevant type-checking logic and existing tests in the repository. Done means the examples type-check with clearly defined, regression-tested semantics.

Written by the indexing model from the issue text.

Description

feature priority-0-high topic-type-variables

Simplified Example:

from typing import TypeVar

_T = TypeVar('_T')
def foo(a: _T = 42) -> _T:  # E: Incompatible types in assignment (expression has type "int", variable has type "_T")
    return a

Real World example is something closer to:

_T = TypeVar('_T')
def noop_parser(x: str) -> str:
    return x

def foo(value: str, parser: Callable[[str], _T] = noop_parser) -> _T:
    return parser(value)
Dominant language
Python
Stars
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Forks
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Avg merge
1d 18h
Merged PRs (30d)
54

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