(No AI or LLMs were used in finding or writing up this issue.)
In Python and passing other type checkers (e.g. pyright 1.1.414 and pyrefly 1.3.1) it is ok to subclass a type passed within the *args variadic argument which is typed as Any, but mypy 2.3.1 and recent master seems to reject this.
E.g. the following Python program runs fine and typechecks via pyright 1.1.414 and pyrefly 1.3.1
(btw if you have an objection to this on the basis of the intermediate x variable, please also see the example pair at the end of the issue showing mypy will allow a keyword arg for a type to subclass but not an element of *args without an intermediate variable)
https://mypy-play.net/?gist=caa19ef606afd2b352e2e5e3fa608bca
from typing import Any
def fun_varargs_element_to_type_used_for_inherit(
*args: *tuple[int, Any],
) -> Any:
# not allowed by mypy but allowed by pyright and others
x = args[1]
class Sub(x): pass
return Sub()
s = fun_varargs_element_to_type_used_for_inherit(1, str)
print('isinstance(s, str): ', isinstance(s, str))
Per pyright 1.1.414 and pyrefly 1.3.1 it typechecks and in python 3.13.2 (and 3.12 see gist above) runs and produces valid output:
(venv) [liveuser@localhost-live mypy]$ pyright ~/examplecomplex2.py
0 errors, 0 warnings, 0 informations
(venv) [liveuser@localhost-live mypy]$ python ~/examplecomplex2.py
isinstance(s, str): True
But mypy 2.3.1 rejects it (see mypy playground link and associated gist https://mypy-play.net/?gist=caa19ef606afd2b352e2e5e3fa608bca ) .
Here are some unit test cases showing this behavior as well:
[case testVarargsArgumentTypeCheckingUsingArgumentTypeFromVarArgsForInheritanceExplicitCastOk]
# flags: --python-version 3.12
from typing import Any
def fun_explicit_cast_varargs_element_to_type_used_for_inherit(
*args: *tuple[int, Any],
):
# allowed
x : type = args[1]
class Sub(x): pass
fun_explicit_cast_varargs_element_to_type_used_for_inherit(1, str)
[builtins fixtures/tuple.pyi]
[case testVarargsArgumentTypeCheckingUsingArgumentTypeFromVarArgsForInheritance-xfail]
# flags: --python-version 3.12
from typing import Any
def fun_varargs_element_to_type_used_for_inherit(
*args: *tuple[int, Any],
):
# not allowed by mypy but allowed by pyright and others
x = args[1]
class Sub(x): pass
fun_varargs_element_to_type_used_for_inherit(1, str)
[builtins fixtures/tuple.pyi]
The first case passes, the second xfails (I marked it expected fail for now per current behavior) when it should pass it seems.
Note that passing a type via an Any typed keyword parameter is OK-ed by mypy:
https://mypy-play.net/?gist=9ff16e8bc2f873f3fe5d0dcb4ff8baeb
[liveuser@localhost-live mypy]$ cat ~/examplecomplex3.py
from typing import Any
def fun_keyword_parameter_to_type_used_for_inherit(
*args: *tuple[int, Any],
keywordarg: Any
) -> Any:
class Sub(keywordarg): pass
return Sub()
s = fun_keyword_parameter_to_type_used_for_inherit(1, 1, keywordarg=str)
print('isinstance(s, str): ', isinstance(s, str))
[liveuser@localhost-live mypy]$ python ~/examplecomplex3.py
isinstance(s, str): True
[liveuser@localhost-live mypy]$ mypy ~/examplecomplex3.py
Success: no issues found in 1 source file
But using args[1] (an element of the *args tuple directly) when Any typed to pass a type for subclassing is not allowed by mypy, while it is allowed by pyright 1.1.414 and pyrefly 1.3.1 and python:
https://mypy-play.net/?gist=d12a1ef7f71677f150d09200fadf1958
[liveuser@localhost-live mypy]$ cat ~/examplecomplex4.py
from typing import Any
def fun_varargs_element_to_type_used_for_inherit(
*args: *tuple[int, Any],
) -> Any:
# not allowed by mypy but allowed by pyright and others
class Sub(args[1]): pass
return Sub()
s = fun_varargs_element_to_type_used_for_inherit(1, str)
print('isinstance(s, str): ', isinstance(s, str))
[liveuser@localhost-live mypy]$ python ~/examplecomplex4.py
isinstance(s, str): True
[liveuser@localhost-live mypy]$ pyright ~/examplecomplex4.py
0 errors, 0 warnings, 0 informations
[liveuser@localhost-live mypy]$ mypy ~/examplecomplex4.py
/home/liveuser/examplecomplex4.py:7: error: Variable "args" is not valid as a type [valid-type]
/home/liveuser/examplecomplex4.py:7: note: See https://mypy.readthedocs.io/en/stable/common_issues.html#variables-vs-type-aliases
/home/liveuser/examplecomplex4.py:7: error: Invalid base class "args" [misc]
/home/liveuser/examplecomplex4.py:7: error: Invalid type: try using Literal[1] instead? [valid-type]
Found 3 errors in 1 file (checked 1 source file)
(Note the above typechecks fine in pyright 1.1.414 and pyrefly 1.3.1 )
- Mypy version used: 2.3.1 (and recent master)
- Mypy command-line flags: None
- Mypy configuration options from
mypy.ini (and other config files): defaults
- Python version used: 3.12 and 3.13.2
Thanks so much for building this tool!
(No AI or LLMs were used in finding or writing up this issue.)
In Python and passing other type checkers (e.g. pyright 1.1.414 and pyrefly 1.3.1) it is ok to subclass a type passed within the
*argsvariadic argument which is typed as Any, but mypy 2.3.1 and recent master seems to reject this.E.g. the following Python program runs fine and typechecks via pyright 1.1.414 and pyrefly 1.3.1
(btw if you have an objection to this on the basis of the intermediate
xvariable, please also see the example pair at the end of the issue showing mypy will allow a keyword arg for a type to subclass but not an element of*argswithout an intermediate variable)https://mypy-play.net/?gist=caa19ef606afd2b352e2e5e3fa608bca
Per pyright 1.1.414 and pyrefly 1.3.1 it typechecks and in python 3.13.2 (and 3.12 see gist above) runs and produces valid output:
But mypy 2.3.1 rejects it (see mypy playground link and associated gist https://mypy-play.net/?gist=caa19ef606afd2b352e2e5e3fa608bca ) .
Here are some unit test cases showing this behavior as well:
The first case passes, the second xfails (I marked it expected fail for now per current behavior) when it should pass it seems.
Note that passing a type via an Any typed keyword parameter is OK-ed by mypy:
https://mypy-play.net/?gist=9ff16e8bc2f873f3fe5d0dcb4ff8baeb
But using
args[1](an element of the*argstuple directly) when Any typed to pass a type for subclassing is not allowed by mypy, while it is allowed by pyright 1.1.414 and pyrefly 1.3.1 and python:https://mypy-play.net/?gist=d12a1ef7f71677f150d09200fadf1958
(Note the above typechecks fine in pyright 1.1.414 and pyrefly 1.3.1 )
mypy.ini(and other config files): defaultsThanks so much for building this tool!