Fix os.ftruncate support in fake_os.

CHANGES.md

@@ -15,6 +15,9 @@ The released versions correspond to PyPI releases.
 
 * fixed a crash if the stack limit was set to a low value
   (see [#1313](https://github.com/pytest-dev/pyfakefs/issues/1313))
+* fixed `os.ftruncate` to inspect the `FakeFileWrapper` returned by
+  `get_open_file(fd)`
+  (see [#1331](https://github.com/pytest-dev/pyfakefs/issues/1331))
 
 ## [Version 6.2.0](https://pypi.python.org/pypi/pyfakefs/6.2.0) (2026-04-12)
 

pyfakefs/fake_os.py

@@ -1019,11 +1019,10 @@ def ftruncate(self, fd: int, length: int) -> None:
         Raises:
             OSError: if the file descriptor is invalid
         """
-        file_object = self.filesystem.get_open_file(fd).get_object()
-        if isinstance(file_object, FakeFileWrapper):
-            file_object.size = length
-        else:
+        file_handle = self.filesystem.get_open_file(fd)
+        if not isinstance(file_handle, FakeFileWrapper):
             self.filesystem.raise_os_error(errno.EBADF)
+        file_handle.get_object().size = length
 
     def access(
         self,

pyfakefs/tests/fake_os_test.py

@@ -2974,7 +2974,7 @@ def test_ftruncate(self):
         self.create_file(file_path, contents="0123456789012345")
 
         fd = self.os.open(file_path, os.O_RDWR)
-        self.os.truncate(fd, 10)
+        self.os.ftruncate(fd, 10)
         self.assertEqual(10, self.os.stat(file_path).st_size)
         with self.open(file_path, encoding="utf8") as f:
             self.assertEqual("0123456789", f.read())