Mypy see invalid syntax about valid syntax in recent Python version

[buxx]

Bug Report

Mypy report a syntax error what is not in Python3.12+ while --python-version arg is used.

To Reproduce

This reproduction steps use uv to make python 3.12 install easier but got same result with standard tools.

➜  cd /tmp
➜  uv venv --python 3.12.6 venv                                              
Using Python 3.12.6
Creating virtual environment at: venv
Activate with: source venv/bin/activate
➜  . venv/bin/activate
(venv) ➜ uv pip install mypy
Resolved 3 packages in 139ms
Installed 3 packages in 22ms
 + mypy==1.11.2
 + mypy-extensions==1.0.0
 + typing-extensions==4.12.2
(venv) ➜  /tmp echo 'import typing               
import enum

class MyEnum(enum.Enum):
    ONE = "ONE"

    def name(self) -> str:
        match self:
            case self.ONE:
                return "1"
            case _:
                typing.assert_never(self)
' > test.py
(venv) ➜ mypy --python-version=3.12 test.py 
test.py:9: error: invalid syntax; you likely need to run mypy using Python 3.12 or newer  [syntax]
Found 1 error in 1 file (errors prevented further checking)

Expected Behavior

mypy don't see syntax error at line 9.

Actual Behavior

mypy see syntax error at line 9.

Your Environment

• Mypy version used: 1.11.2
• Mypy command-line flags: --python-version=3.12
• Mypy configuration options from mypy.ini (and other config files): none
• Python version used: 3.12.6

[JelleZijlstra]

You are likely running a mypy executable that is based on an earlier version of Python. Try running which mypy to figure out where it came from.

[hauntsaninja]

Yes, or try hash -r before running mypy.

Although I don't actually see what the syntax error is here, so I think there's also something not right with the repro steps provided and can't repro any issue on any version combination myself

[JelleZijlstra]

Probably the match statement, meaning they're running mypy with Python 3.9 or earlier.

[hauntsaninja]

Ah I forgot that Python 3.9 ever existed, was just like I don't see new generic syntax!

[hauntsaninja]

When I made this error message better, I wanted to also include the user's current version. But it made the testing complicated, so I didn't :-/

[buxx]

Hi, there is result of wich :

➜  /tmp uv venv --python 3.12.6 venv                 
Using Python 3.12.6
Creating virtual environment at: venv
Activate with: source venv/bin/activate
➜  /tmp . venv/bin/activate
(venv) ➜  /tmp uv pip install mypy
Resolved 3 packages in 150ms
Installed 3 packages in 20ms
 + mypy==1.11.2
 + mypy-extensions==1.0.0
 + typing-extensions==4.12.2
(venv) ➜  /tmp mypy --version
mypy 1.11.2 (compiled: yes)
(venv) ➜  /tmp which mypy
/home/user/.local/bin/mypy

the mypy venv success :

(venv) ➜  /tmp venv/bin/mypy test.py 
Success: no issues found in 1 source file

So, executed mypy is not the venv mypy ... So two point :

• (which not concern python/mypy source code) Why venv mypy is not used in prior
• mypy, when executed with old python (how to confirm taht ?) should work about indicated --python-version or fail if it can't

[hauntsaninja]

Like I said before, try hash -r

mypy is warning about old Python, that's what the "you likely need to run mypy using Python 3.12 or newer" is about.

[buxx]

Like I said before, try hash -r

How should I use hash -r ?

mypy is warning about old Python, that's what the "you likely need to run mypy using Python 3.12 or newer" is about.

Oh ! I now understand properly this warning ... !

[JelleZijlstra]

How should I use hash -r ?

You should run this command. It clears your shell's cache that caches the location where it finds the mypy executable.