Add checks to avoid patching debugger modules

- added Patcher.RUNTIME_SKIPMODULES to allow to skip
  modules whose name starts with specific strings,
  if the related debuuger is loaded
- only done for debugger used in PyCharm and VSCode

Author: mrbean-bremen

CHANGES.md

@@ -17,6 +17,8 @@ The released versions correspond to PyPI releases.
 ### Enhancements
 * added some support for loading fake modules in `AUTO` patch mode
   using `importlib.import_module` (see [#1079](../../issues/1079))
+* added some support to avoid patching debugger related modules
+  (see [#1083](../../issues/1083))
 
 ### Performance
 * avoid reloading `tempfile` in Posix systems

pyfakefs/fake_filesystem_unittest.py

@@ -514,6 +514,13 @@ class Patcher:
         SKIPMODULES.add(posixpath)
         SKIPMODULES.add(fcntl)
 
+    # a list of modules detected at run-time
+    # each tool defines one or more module name prefixes for modules to be skipped
+    RUNTIME_SKIPMODULES = {
+        "pydevd": ["_pydevd_", "pydevd", "_pydev_"],  # Python debugger (PyCharm/VSCode)
+        "_jb_runner_tools": ["_jb_"],  # JetBrains tools
+    }
+
     # caches all modules that do not have file system modules or function
     # to speed up _find_modules
     CACHED_MODULES: Set[ModuleType] = set()
@@ -1045,10 +1052,26 @@ def patch_functions(self) -> None:
                 self._stubs.smart_set(module, name, attr)
 
     def patch_modules(self) -> None:
+        skip_prefix_list = []
+        for rt_skip_module, prefixes in self.RUNTIME_SKIPMODULES.items():
+            if rt_skip_module in sys.modules:
+                skip_prefix_list.extend(prefixes)
+        skip_prefixes = tuple(skip_prefix_list)
+
         assert self._stubs is not None
         for name, modules in self.FS_MODULES.items():
             for module, attr in modules:
-                self._stubs.smart_set(module, name, self.fake_modules[attr])
+                try:
+                    if not skip_prefixes or not module.__name__.startswith(
+                        skip_prefixes
+                    ):
+                        self._stubs.smart_set(module, name, self.fake_modules[attr])
+                    elif attr in self.unfaked_modules:
+                        self._stubs.smart_set(module, name, self.unfaked_modules[attr])
+                except Exception:
+                    # handle the rare case that a module has no __name__
+                    pass
+
         for name, modules in self.SKIPPED_FS_MODULES.items():
             for module, attr in modules:
                 if attr in self.unfaked_modules:

pyfakefs/tests/fake_filesystem_unittest_test.py

@@ -456,6 +456,36 @@ def test_path_succeeds(self):
         pyfakefs.tests.import_as_example.return_this_file_path()
 
 
+class RuntimeSkipModuleTest(fake_filesystem_unittest.TestCase):
+    """Emulates skipping a module using RUNTIME_SKIPMODULES.
+    Not all functionality implemented for skip modules will work here."""
+
+    def setUp(self):
+        Patcher.RUNTIME_SKIPMODULES.update(
+            {"pyfakefs.tests.import_as_example": ["pyfakefs.tests.import_"]}
+        )
+        self.setUpPyfakefs()
+
+    def tearDown(self):
+        del self.patcher.RUNTIME_SKIPMODULES["pyfakefs.tests.import_as_example"]
+
+    def test_fake_path_does_not_exist1(self):
+        self.fs.create_file("foo")
+        self.assertFalse(pyfakefs.tests.import_as_example.check_if_exists1("foo"))
+
+    def test_fake_path_does_not_exist2(self):
+        self.fs.create_file("foo")
+        self.assertFalse(pyfakefs.tests.import_as_example.check_if_exists2("foo"))
+
+    def test_fake_path_does_not_exist3(self):
+        self.fs.create_file("foo")
+        self.assertFalse(pyfakefs.tests.import_as_example.check_if_exists3("foo"))
+
+    def test_fake_path_does_not_exist4(self):
+        self.fs.create_file("foo")
+        self.assertFalse(pyfakefs.tests.import_as_example.check_if_exists4("foo"))
+
+
 class FakeExampleModule:
     """Used to patch a function that uses system-specific functions that
     cannot be patched automatically."""