Incorrectly narrows `type` from signature annotation to `Never`
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Description
Bug Report
When taking the parameter type from the signature, mypy incorrectly thinks that the annotation of type <class 'type'> is the actual type of the variable.
To Reproduce
In the following example, process_signature extracts the type of input from the signature of a function and then tries to match it to int or float. However the type of input_type is <class 'type'> not int or float. Therefore, when the code is run assert_never is reached.
from typing import Callable, assert_never, reveal_type
from inspect import signature
def takes_int(x: int) -> None:
print(f"{x} is a int")
def process_signature(
func: Callable[[int], None] | Callable[[float], None]
) -> None:
input_type = signature(func).parameters['x'].annotation
reveal_type(input_type) # Runtime type is 'type' => should reach `case _:`
match input_type:
case int():
print("This is an int")
case float():
print("This is a float")
case _:
assert_never(input_type) # this is reached during runtime
process_signature(takes_int)
Expected Behavior
mypy to complain that input to assert_never is <class 'type'> but expected Never.
Actual Behavior
mypy does not complain.
Your Environment
- Mypy version used:
1.19.1 - Mypy command-line flags: None
- Mypy configuration options from
mypy.ini(and other config files): None - Python version used:
3.13.7
Contributor guide
First steps
- Read the whole issue, then the project's contributing guide.
- Comment on the issue to say you are picking it up — it saves two people doing the same work.
- Fork the repository and make your change on a branch.
- Open a pull request that references the issue number.
Research direction
Start by running the provided reproducer with mypy 1.19.1 and inspect the revealed type for input_type. Trace how the match cases narrow the annotation from inspect.signature through assert_never. Done means mypy reports that the remaining input_type is type rather than treating the wildcard case as Never.
Written by the indexing model from the issue text.
Assessment
- Tech stack
- python
- Domain
- devtools
- Issue type
- Bug
- Difficulty
- 4/5
- Estimated time
- 3-5 days
- Activity status
- Stale
- Clarity
- Mostly clear
- Newbie friendliness
- 45/100