python / python/mypy

`bool` is available even if `*constraints` of `TypeVar` is the exact types `int` and `float`

Open
#20,314 4 comments 0 reactions 0 assignees View on GitHub

Nobody has claimed this yet.

bug topic-type-variables
Dominant language
Python
Stars
20.6k
Forks
3.3k
PR merge metrics
PR metrics pending

Description

*Memo:

  • mypy --strict test.py
  • mypy 1.18.2
  • Python 3.14.0
  • Windows 11

The doc says 'Must be exactly str or bytes' about *constraints to make difference between bound and *constraints as shown below:

*Memo:

  • From my understanding:
    • Setting bound, the type and the subtypes are available.
    • Setting *constraints, only the exact types are available but not the subtypes.

T = TypeVar('T') # Can be anything
S = TypeVar('S', bound=str) # Can be any subtype of str
A = TypeVar('A', str, bytes) # Must be exactly str or bytes

But bool is available even if *constraints is the exact types int and float as shown below:

<With TypeVar (Old syntax)>:
from typing import TypeVar, Generic

T = TypeVar('T', int, float)

class MyCls(Generic[T]):
    num: T

    def __init__(self, x: T) -> None:
        self.num = x

           # ↓↓↓↓↓↓↓↓↓↓↓↓
mycls = MyCls[bool](True)
# No error
<Without TypeVar (New syntax)>:
class MyCls[T: (int, float)]:
    num: T

    def __init__(self, x: T) -> None:
        self.num = x

           # ↓↓↓↓↓↓↓↓↓↓↓↓
mycls = MyCls[bool](True)
# No error

Contributor guide

Open the contributing guide

First steps

  1. Read the whole issue, then the project's contributing guide.
  2. Comment on the issue to say you are picking it up — it saves two people doing the same work.
  3. Fork the repository and make your change on a branch.
  4. Open a pull request that references the issue number.

Research direction

Start by running the supplied mypy --strict test.py reproduction with mypy 1.18.2 and compare the old and new TypeVar syntaxes. Trace the type-checking behavior for constrained int/float TypeVars; done means the implementation consistently applies the documented constraint semantics and regression coverage captures the result.

Written by the indexing model from the issue text.

Assessment

Tech stack
python
Domain
devtools
Issue type
Bug
Difficulty
3/5
Estimated time
1-2 days
Activity status
Stale
Clarity
Mostly clear
Newbie friendliness
45/100

Get new issues in your inbox

A short digest of beginner-friendly GitHub issues.