pkgutil.walk_packages uses mutable object as a default parameter in internal seen function

[asai95]

Bug report

Bug description:

As the title says, pkgutil.walk_packages is using mutable default (a dictionary) to remember modules it already seen.
Code link.

This may lead to hard-to-track bugs, when you try to import a package with the same name, but in different folder.

Example code where this issue could break stuff (I'm actually using something similar in my tests):

import importlib
import pkgutil
import sys
import tempfile
from pathlib import Path


DUMMY_MODULE = """
def hello_world():
    return "Hello, World!"
"""


class MyLoader:
    def __init__(self, path):
        self.path = path
        self.available_modules = {}

    def load_module(self, fullname):
        if fullname in self.available_modules:
            return self.available_modules[fullname]

    def discover_modules(self):
        sys.path.append(str(self.path))
        for _, module_name, is_pkg in pkgutil.walk_packages([self.path]):
            if not is_pkg:
                module = importlib.import_module(module_name)
                self.available_modules[module_name] = module
        sys.path.remove(str(self.path))


def do_test():
    print("Running test")
    with tempfile.TemporaryDirectory() as tmp_dir:
        tmp_path = Path(tmp_dir)
        package_name = "my_package"
        package_path = tmp_path / package_name
        package_path.mkdir()
        module_path = package_path / "my_module.py"
        init_path = package_path / "__init__.py"
        init_path.touch()
        with module_path.open("w") as f:
            f.write(DUMMY_MODULE)

        loader = MyLoader(path=tmp_path)
        loader.discover_modules()
        module = loader.load_module("my_package.my_module")
        assert module.hello_world() == "Hello, World!"
    print("Test passed")


if __name__ == "__main__":
    do_test()
    do_test()  # This will fail

CPython versions tested on:

3.13

Operating systems tested on:

Linux

[picnixz]

1. What is the expected output? how does it fail? (sorry I'm on mobile and I can't really reproduce this easily).
2. How do you suggest changing the code? The reason why it keeps track of seen modules is to avoid recursion issues (i.e. it can be regarded as a recursive guard).

[asai95]

Sorry, when writing an answer I understood that the problem is something else so I'll close the issue. But thanks @picnixz for your swift reply!