Python 3.13 test failure: tests/config/loader/test_str_convert.py::test_str_convert_ok_py39[1,2-value1-Optional] - TypeError: issubclass() arg 1 must be a class

[mgorny]

Issue

When running the test suite on Python 3.13.0b1, I'm getting the following test failure:

____________________________________________ test_str_convert_ok_py39[1,2-value1-Optional] ____________________________________________

raw = '1,2', value = ['1', '2'], of_type = typing.Optional[list[str]]

    @pytest.mark.parametrize(
        ("raw", "value", "of_type"),
        [
            ("", None, Optional[list[str]]),
            ("1,2", ["1", "2"], Optional[list[str]]),
        ],
    )
    def test_str_convert_ok_py39(raw: str, value: Any, of_type: type[Any]) -> None:
>       result = StrConvert().to(raw, of_type, None)

of_type    = typing.Optional[list[str]]
raw        = '1,2'
value      = ['1', '2']

tests/config/loader/test_str_convert.py:71: 
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _
../tox-4.15.1-python3_13/install/usr/lib/python3.13/site-packages/tox/config/loader/convert.py:32: in to
    return self._to_typing(raw, of_type, factory)
        factory    = None
        from_module = 'typing'
        of_type    = typing.Optional[list[str]]
        raw        = '1,2'
        self       = <tox.config.loader.str_convert.StrConvert object at 0x7fdd060385a0>
../tox-4.15.1-python3_13/install/usr/lib/python3.13/site-packages/tox/config/loader/convert.py:79: in _to_typing
    result = self.to(raw, new_type, factory)
        args       = (list[str], <class 'NoneType'>)
        factory    = None
        new_type   = list[str]
        none       = <class 'NoneType'>
        of_type    = typing.Optional[list[str]]
        origin     = typing.Union
        raw        = '1,2'
        result     = <object object at 0x7fdd090df730>
        self       = <tox.config.loader.str_convert.StrConvert object at 0x7fdd060385a0>
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _

self = <tox.config.loader.str_convert.StrConvert object at 0x7fdd060385a0>, raw = '1,2', of_type = list[str], factory = None

    def to(self, raw: T, of_type: type[V], factory: Factory[V]) -> V:  # noqa: PLR0911
        """
        Convert given raw type to python type.
    
        :param raw: the raw type
        :param of_type: python type
        :param factory: factory method to build the object
        :return: the converted type
        """
        from_module = getattr(of_type, "__module__", None)
        if from_module in {"typing", "typing_extensions"}:
            return self._to_typing(raw, of_type, factory)
>       if issubclass(of_type, Path):
E       TypeError: issubclass() arg 1 must be a class

factory    = None
from_module = 'builtins'
of_type    = list[str]
raw        = '1,2'
self       = <tox.config.loader.str_convert.StrConvert object at 0x7fdd060385a0>

../tox-4.15.1-python3_13/install/usr/lib/python3.13/site-packages/tox/config/loader/convert.py:33: TypeError

Apparently, this is intentional change: python/cpython#101162

Environment

Provide at least:

• OS: Gentoo Linux amd64

Output of running tox

n/a

Minimal example

n/a

[hroncok]

This ugly diff makes it work:

diff --git a/src/tox/config/loader/convert.py b/src/tox/config/loader/convert.py
index 9379c6ea..66ad266d 100644
--- a/src/tox/config/loader/convert.py
+++ b/src/tox/config/loader/convert.py
@@ -30,6 +30,10 @@ class Convert(ABC, Generic[T]):
         from_module = getattr(of_type, "__module__", None)
         if from_module in {"typing", "typing_extensions"}:
             return self._to_typing(raw, of_type, factory)
+        # python does not allow use of parametrized generics with isinstance,
+        # so we need to check for them.
+        if hasattr(typing, "GenericAlias") and isinstance(of_type, typing.GenericAlias):
+            return list(self.to_list(raw, of_type=of_type))  # type: ignore[return-value]
         if issubclass(of_type, Path):
             return self.to_path(raw)  # type: ignore[return-value]
         if issubclass(of_type, bool):
@@ -40,10 +44,6 @@ class Convert(ABC, Generic[T]):
             return self.to_env_list(raw)  # type: ignore[return-value]
         if issubclass(of_type, str):
             return self.to_str(raw)  # type: ignore[return-value]
-        # python does not allow use of parametrized generics with isinstance,
-        # so we need to check for them.
-        if hasattr(typing, "GenericAlias") and isinstance(of_type, typing.GenericAlias):
-            return list(self.to_list(raw, of_type=of_type))  # type: ignore[return-value]
         if isinstance(raw, of_type):  # already target type no need to transform it
             # do it this late to allow normalization - e.g. string strip
             return raw
diff --git a/src/tox/config/loader/str_convert.py b/src/tox/config/loader/str_convert.py
index 63fc41de..f2888a13 100644
--- a/src/tox/config/loader/str_convert.py
+++ b/src/tox/config/loader/str_convert.py
@@ -6,6 +6,7 @@ import shlex
 import sys
 from itertools import chain
 from pathlib import Path
+import typing
 from typing import TYPE_CHECKING, Any, Iterator
 
 from tox.config.loader.convert import Convert
@@ -28,7 +29,7 @@ class StrConvert(Convert[str]):
 
     @staticmethod
     def to_list(value: str, of_type: type[Any]) -> Iterator[str]:
-        splitter = "\n" if issubclass(of_type, Command) or "\n" in value else ","
+        splitter = "\n" if (not isinstance(of_type, getattr(typing, "GenericAlias", type(None))) and issubclass(of_type, Command)) or "\n" in value else ","
         splitter = splitter.replace("\r", "")
         for token in value.split(splitter):
             value = token.strip()

It makes sure that:

• the GenericAlias check in convert.py happens first
• the GenericAlias check is added to str_convert.py

The change in str_convert could probably be split into a function of its own.

[hroncok]

I'd submit a PR, but I wonder where else this is used, possibly not covered by tests?

[hroncok]

Also, if I need a is_generic_alias(t) function, where would be the best place for it to be defined?

[gaborbernat]

Up to you 👍 I would keep it private and close to where it is used.