Issue warning for ignored arguments in nested call

- if a fake filesystem is created inside an existing fake fs,
  the existing fs is reused, and any arguments ignored

Author: mrbean-bremen

CHANGES.md

@@ -13,6 +13,12 @@ The released versions correspond to PyPI releases.
   the real filesystem behavior
 * remove support for Python versions before 3.10 (if needed, patches may be backported to the 5.x branch)
 
+## Unreleased
+
+## Changes
+* a warning is now issued if trying to create a nested fake filesystem with custom arguments
+  (custom arguments are ignored in this case, as the existing fake filesystem is used)
+
 ## [Version 5.9.2](https://pypi.python.org/pypi/pyfakefs/5.9.2) (2025-07-30)
 Fixes interaction with pytest.
 

docs/troubleshooting.rst

@@ -381,7 +381,8 @@ Nested file system fixtures and Patcher invocations
 ``pyfakefs`` does not support nested faked file systems. Instead, it uses reference counting
 on the single fake filesystem instance. That means, if you are trying to create a fake filesystem
 inside a fake filesystem, only the reference count will increase, and any arguments you may pass
-to the patcher or fixture are ignored. Likewise, if you leave a nested fake filesystem,
+to the patcher or fixture are ignored. In this case (e.g. if passing custom arguments to a nested
+invocation), a warning is issued py ``pyfakefs``. Likewise, if you leave a nested fake filesystem,
 only the reference count is decreased and nothing is reverted.
 
 There are some situations where that may happen, probably without you noticing:

pyfakefs/fake_filesystem_unittest.py

@@ -655,6 +655,21 @@ def __init__(
             if self.DOC_REF_COUNT > 0:
                 return
         elif self.REF_COUNT > 0:
+            if (
+                additional_skip_names
+                or modules_to_reload
+                or modules_to_patch
+                or not allow_root_user
+                or not use_known_patches
+                or patch_open_code != PatchMode.OFF
+                or patch_default_args
+                or not use_cache
+                or not use_dynamic_patch
+            ):
+                warnings.warn(
+                    "Nested fake filesystem invocation using custom arguments - "
+                    "using the existing fake filesystem, discarding custom arguments!"
+                )
             return
         if not allow_root_user:
             # set non-root IDs even if the real user is root

pyfakefs/tests/fake_filesystem_unittest_test.py

@@ -71,6 +71,25 @@ def test_context_decorator(self, fake_fs):
             contents = f.read()
         self.assertEqual("test", contents)
 
+    def test_nested_invocation(self):
+        with Patcher() as patcher:
+            patcher.fs.create_file("/foo/bar", contents="test")
+            self.assertTrue(patcher.fs.exists("/foo/bar"))
+            with Patcher() as patcher1:
+                self.assertTrue(patcher1.fs.exists("/foo/bar"))
+                patcher1.fs.create_file("/foo/baz", contents="test")
+                self.assertTrue(patcher1.fs.exists("/foo/baz"))
+            self.assertTrue(patcher.fs.exists("/foo/baz"))
+
+    def test_nested_invocation_with_args(self):
+        with Patcher() as patcher:
+            patcher.fs.create_file("/foo/bar", contents="test")
+            with self.assertWarnsRegex(
+                UserWarning, "Nested fake filesystem invocation using custom arguments"
+            ):
+                with Patcher(allow_root_user=False):
+                    pass
+
 
 class TestPatchfsArgumentOrder(TestCase):
     @patchfs