Windows: ImportError loading conftest due to path casing regression (pytest 5.1.2)

[rowan-stein]

User's report

5.1.2 ImportError while loading conftest (windows import folder casing issues)
5.1.1 works fine. after upgrade to 5.1.2, the path was converted to lower case

Installing collected packages: pytest
  Found existing installation: pytest 5.1.1
    Uninstalling pytest-5.1.1:
      Successfully uninstalled pytest-5.1.1
Successfully installed pytest-5.1.2
PS C:\Azure\KMS\ComponentTest\Python> pytest --collect-only .\PIsys -m smoke
ImportError while loading conftest 'c:\azure\kms\componenttest\python\pisys\conftest.py'.
ModuleNotFoundError: No module named 'python'
PS C:\Azure\KMS\ComponentTest\Python>

Additional context from affected users

• Importing from package with uppercase components (e.g., muepy.imageProcessing.wafer.sawStreets) fails with ModuleNotFoundError: No module named 'muepy.imageprocessing' (note the lowercase p).
• This occurs after upgrading from pytest 5.1.1 to 5.1.2 on Windows 10.

pytest output

(%PREFIX%) %SRC_DIR%>pytest --pyargs muepy
============================= test session starts =============================
platform win32 -- Python 3.6.7, pytest-5.1.2, py-1.8.0, pluggy-0.12.0
rootdir: %SRC_DIR%
collected 0 items / 1 errors

=================================== ERRORS ====================================
________________________ ERROR collecting test session ________________________
..\_test_env\lib\site-packages\_pytest\config\__init__.py:440: in _importconftest
    return self._conftestpath2mod[conftestpath]
E   KeyError: local('c:\\users\\angelo.peronio\\appdata\\local\\continuum\\miniconda3\\envs\\packaging\\conda-bld\\muepy_1567627432048\\_test_env\\lib\\site-packages\\muepy\\imageprocessing\\wafer\\sawstreets\\tests\\conftest.py')

During handling of the above exception, another exception occurred:
..\_test_env\lib\site-packages\_pytest\config\__init__.py:446: in _importconftest
    mod = conftestpath.pyimport()
..\_test_env\lib\site-packages\py\_path\local.py:701: in pyimport
    __import__(modname)
E   ModuleNotFoundError: No module named 'muepy.imageprocessing'

During handling of the above exception, another exception occurred:
..\_test_env\lib\site-packages\py\_path\common.py:377: in visit
    for x in Visitor(fil, rec, ignore, bf, sort).gen(self):
..\_test_env\lib\site-packages\py\_path\common.py:429: in gen
    for p in self.gen(subdir):
..\_test_env\lib\site-packages\py\_path\common.py:429: in gen
    for p in self.gen(subdir):
..\_test_env\lib\site-packages\py\_path\common.py:429: in gen
    for p in self.gen(subdir):
..\_test_env\lib\site-packages\py\_path\common.py:418: in gen
    dirs = self.optsort([p for p in entries
..\_test_env\lib\site-packages\py\_path\common.py:419: in <listcomp>
    if p.check(dir=1) and (rec is None or rec(p))])
..\_test_env\lib\site-packages\_pytest\main.py:606: in _recurse
    ihook = self.gethookproxy(dirpath)
..\_test_env\lib\site-packages\_pytest\main.py:424: in gethookproxy
    my_conftestmodules = pm._getconftestmodules(fspath)
..\_test_env\lib\site-packages\_pytest\config\__init__.py:420: in _getconftestmodules
    mod = self._importconftest(conftestpath)
..\_test_env\lib\site-packages\_pytest\config\__init__.py:454: in _importconftest
    raise ConftestImportFailure(conftestpath, sys.exc_info())
E   _pytest.config.ConftestImportFailure: (local('c:\\users\\angelo.peronio\\appdata\\local\\continuum\\miniconda3\\envs\\packaging\\conda-bld\\muepy_1567627432048\\_test_env\\lib\\site-packages\\muepy\\imageprocessing\\wafer\\sawstreets\\tests\\conftest.py'), (<class 'ModuleNotFoundError'>, ModuleNotFoundError("No module named 'muepy.imageprocessing'",), <traceback object at 0x0000018F0D6C9A48>))
!!!!!!!!!!!!!!!!!!! Interrupted: 1 errors during collection !!!!!!!!!!!!!!!!!!!
============================== 1 error in 1.32s ===============================

Specification (by Emerson Gray)

Root cause:

• Regression introduced by using os.path.normcase in path normalization (PR 5792), which lowercases paths on Windows.
• Lowercased paths are used during conftest import, causing module-name derivation in lowercase and mismatches when conftest imports packages using case-preserved names (e.g., imageProcessing).

Fix strategy:

• Continue using unique_path (normcase) for cache/dedup keys to avoid duplicate conftest loading across differently cased paths.
• Use a case-preserving real path for the actual import operations:
  • In _pytest/config/__init__.py::PytestPluginManager._importconftest, derive pkgpath and call pyimport() using conftestpath.realpath() (case-preserving) while keeping unique_path(conftestpath) only for internal caches.

Proposed code changes:

• File: pytest/src/_pytest/config/__init__.py
• Function: PytestPluginManager._importconftest
• Changes:
  • Replace direct use of unique_path(conftestpath) for pyimport with two variables:
    • conftestpath_key = unique_path(conftestpath) (normcased, for caches)
    • real_conftestpath = conftestpath.realpath() (case-preserving, for import)
  • Use real_conftestpath.pypkgpath() and real_conftestpath.pyimport() for module import.
  • Store mod in _conftestpath2mod keyed by conftestpath_key.
  • Update _dirpath2confmods keyed by conftestpath_key.dirpath().
  • Raise ConftestImportFailure with real_conftestpath for clearer error output.

Regression test:

• File: pytest/testing/test_windows_conftest_import_casing.py
• Behavior:
  • Skip on non-Windows.
  • Create a package imageProcessing/subpkg/helper.py with FOO = 1.
  • In conftest.py, import imageProcessing.subpkg.helper (mixed case) and assert helper.FOO == 1.
  • Add a trivial test to trigger conftest import; ensure collection succeeds.

Backward compatibility:

• Retains deduplication benefits of unique_path while restoring correct module-name derivation on Windows.
• Non-Windows behavior unchanged.

Reproduction steps

1. On Windows, create a package with uppercase letters in directory names.
2. Use a conftest.py that imports a submodule using the mixed-case package name.
3. Run pytest to collect tests; observe ImportError while loading conftest in 5.1.2.
4. With the proposed fix, collection should succeed.