gh-146310: Fix ensurepip to treat empty WHEEL_PKG_DIR as unset

[kimimgo]

Fixes #146310

Path('') resolves to the current working directory (PosixPath('.')), so when WHEEL_PKG_DIR is an empty string (not None), ensurepip searches CWD for wheel files. This can cause unexpected behavior or crashes if CWD contains files matching pip-*.whl.

Changes

• Lib/ensurepip/__init__.py: Add truthiness check so empty strings are treated the same as None
• Lib/test/test_ensurepip.py: Add test verifying empty WHEEL_PKG_DIR does not search CWD

Before/After

# Before: Path('').resolve() -> PosixPath('/current/working/dir')
# Empty string was treated as a valid directory path

# After: Empty string treated as None -> use bundled wheel

• Issue: ensurepip looks for wheels in CWD when WHEEL_PKG_DIR is an empty string #146310

[vstinner]

You may write it as:

Suggested change

	if (_pkg_dir := sysconfig.get_config_var('WHEEL_PKG_DIR')) is not None and _pkg_dir:
	_pkg_dir = sysconfig.get_config_var('WHEEL_PKG_DIR')
	if _pkg_dir:

[vstinner]

This test is not directly related to the fix. I suggest a different test:

    def test_wheel_pkg_dir_none(self):
        for value in ('', None):
            with unittest.mock.patch('sysconfig.get_config_var',
                                     return_value=value) as get_config_var:
                module = import_helper.import_fresh_module('ensurepip')
                self.assertIsNone(module._WHEEL_PKG_DIR)
                get_config_var.assert_called_once_with('WHEEL_PKG_DIR')

with from test.support import import_helper at top level.

[vstinner]

This NEWS entry sounds too technical. I suggest to describe instead the old behavior:

The :mod:`ensurepip` module no longer looks for `pip-*.whl` wheel packages in the current directory.

[kimimgo]

Thanks @vstinner! Updated all three points:

• Simplified to if _pkg_dir:
• Replaced test with import_fresh_module covering both '' and None
• Reworded NEWS entry