fix: add more isolation from os-module mocking

CHANGES.rst

@@ -27,9 +27,14 @@ Unreleased
   statements changed how they traced.  This affected whether people saw the
   fix for `issue 1880`_.
 
+- fix: isolate our code more from mocking in the os module that in rare cases
+  can cause `bizarre behavior <pytest-cov-666_>`_.
+
 - refactor: some code unreachable code paths in parser.py were changed to
   asserts.  If you encounter any of these, please let me know!
 
+.. _pytest-cov-666: https://github.com/pytest-dev/pytest-cov/issues/666
+
 
 .. start-releases
 

coverage/core.py

@@ -12,6 +12,7 @@
 from coverage import env
 from coverage.disposition import FileDisposition
 from coverage.exceptions import ConfigError
+from coverage.misc import isolate_module
 from coverage.pytracer import PyTracer
 from coverage.sysmon import SysMonitor
 from coverage.types import (
@@ -21,6 +22,8 @@
 )
 
 
+os = isolate_module(os)
+
 try:
     # Use the C extension code when we can, for speed.
     from coverage.tracer import CTracer, CFileDisposition

coverage/inorout.py

@@ -9,6 +9,7 @@
 import inspect
 import itertools
 import os
+import os.path
 import platform
 import re
 import sys
@@ -26,7 +27,7 @@
 from coverage.exceptions import CoverageException, PluginError
 from coverage.files import TreeMatcher, GlobMatcher, ModuleMatcher
 from coverage.files import prep_patterns, find_python_files, canonical_filename
-from coverage.misc import sys_modules_saved
+from coverage.misc import isolate_module, sys_modules_saved
 from coverage.python import source_for_file, source_for_morf
 from coverage.types import TFileDisposition, TMorf, TWarnFn, TDebugCtl
 
@@ -57,6 +58,8 @@
         pass
 
 
+os = isolate_module(os)
+
 def canonical_path(morf: TMorf, directory: bool = False) -> str:
     """Return the canonical path of the module or file `morf`.
 

coverage/parser.py

@@ -23,10 +23,12 @@
 from coverage.bytecode import code_objects
 from coverage.debug import short_stack
 from coverage.exceptions import NoSource, NotPython
-from coverage.misc import nice_pair
+from coverage.misc import isolate_module, nice_pair
 from coverage.phystokens import generate_tokens
 from coverage.types import TArc, TLineNo
 
+os = isolate_module(os)
+
 
 class PythonParser:
     """Parse code to find executable lines, excluded lines, etc.

coverage/sysmon.py

@@ -23,6 +23,7 @@
 )
 
 from coverage.debug import short_filename, short_stack
+from coverage.misc import isolate_module
 from coverage.types import (
     AnyCallable,
     TArc,
@@ -36,6 +37,8 @@
     TWarnFn,
 )
 
+os = isolate_module(os)
+
 # pylint: disable=unused-argument
 
 LOG = False

coverage/tomlconfig.py

@@ -13,9 +13,10 @@
 
 from coverage import env
 from coverage.exceptions import ConfigError
-from coverage.misc import import_third_party, substitute_variables
+from coverage.misc import import_third_party, isolate_module, substitute_variables
 from coverage.types import TConfigSectionOut, TConfigValueOut
 
+os = isolate_module(os)
 
 if env.PYVERSION >= (3, 11, 0, "alpha", 7):
     import tomllib      # pylint: disable=import-error