Do not set size to 0 on setting initial content

- caused file system file to be wrong for non-empty files
- tests: add contextmanager version of assert_raises_os_error
- fixes pytest-dev#660

CHANGES.md

@@ -1,6 +1,12 @@
 # pyfakefs Release Notes
 The released versions correspond to PyPi releases.
 
+## Unreleased
+
+### Fixes
+* correctly handle file system space for files opened in write mode
+  (see [#660](../../issues/660))
+
 ## [Version 4.5.4](https://pypi.python.org/pypi/pyfakefs/4.5.4) (2022-01-12)
 Minor bugfix release.
 

pyfakefs/fake_filesystem.py

@@ -439,8 +439,6 @@ def set_initial_contents(self, contents: AnyStr) -> bool:
         current_size = self.st_size or 0
         self.filesystem.change_disk_usage(
             st_size - current_size, self.name, self.st_dev)
-        if self._byte_contents:
-            self.size = 0
         self._byte_contents = byte_contents
         self.st_size = st_size
         self.epoch += 1