Add support for os.truncate and os.ftruncate

- fixes #545

CHANGES.md

@@ -6,6 +6,8 @@ The released versions correspond to PyPi releases.
 #### Fixes
   * suppress deprecation warnings while collecting modules
    (see [#542](../../issues/542))
+  * add support for `os.truncate` and `os.ftruncate`
+   (see [#545](../../issues/545))
 
 ## [Version 4.1.0](https://pypi.python.org/pypi/pyfakefs/4.1.0)
 

pyfakefs/fake_filesystem.py

@@ -4136,6 +4136,38 @@ def _path_with_dir_fd(self, path, fct, dir_fd):
                         dir_fd).get_object().path, path)
         return path
 
+    def truncate(self, path, length):
+        """Truncate the file corresponding to path, so that it is
+         length bytes in size. If length is larger than the current size,
+         the file is filled up with zero bytes.
+
+        Args:
+            path: (str or int) Path to the file, or an integer file
+                descriptor for the file object.
+            length: (int) Length of the file after truncating it.
+
+        Raises:
+            OSError: if the file does not exist or the file descriptor is
+                invalid.
+         """
+        file_object = self.filesystem.resolve(path, allow_fd=True)
+        file_object.size = length
+
+    def ftruncate(self, fd, length):
+        """Truncate the file corresponding to fd, so that it is
+         length bytes in size. If length is larger than the current size,
+         the file is filled up with zero bytes.
+
+        Args:
+            fd: (int) File descriptor for the file object.
+            length: (int) Maximum length of the file after truncating it.
+
+        Raises:
+            OSError: if the file descriptor is invalid
+         """
+        file_object = self.filesystem.get_open_file(fd).get_object()
+        file_object.size = length
+
     def access(self, path, mode, *, dir_fd=None, follow_symlinks=True):
         """Check if a file exists and has the specified permissions.
 

pyfakefs/tests/fake_os_test.py

@@ -2708,6 +2708,52 @@ def test_write_to_read_fd(self):
         self.os.close(read_fd)
         self.os.close(write_fd)
 
+    def test_truncate(self):
+        file_path = self.make_path('foo', 'bar')
+        self.create_file(file_path, contents='012345678901234567')
+        self.os.truncate(file_path, 10)
+        with self.open(file_path) as f:
+            self.assertEqual('0123456789', f.read())
+
+    def test_truncate_non_existing(self):
+        self.assert_raises_os_error(errno.ENOENT, self.os.truncate, 'foo', 10)
+
+    def test_truncate_to_larger(self):
+        file_path = self.make_path('foo', 'bar')
+        self.create_file(file_path, contents='0123456789')
+        fd = self.os.open(file_path, os.O_RDWR)
+        self.os.truncate(fd, 20)
+        self.assertEqual(20, self.os.stat(file_path).st_size)
+        with self.open(file_path) as f:
+            self.assertEqual('0123456789' + '\0' * 10, f.read())
+
+    def test_truncate_with_fd(self):
+        if os.truncate not in os.supports_fd:
+            self.skip_real_fs()
+        self.assert_raises_os_error(errno.EBADF, self.os.ftruncate, 50, 10)
+        file_path = self.make_path('some_file')
+        self.create_file(file_path, contents='01234567890123456789')
+
+        fd = self.os.open(file_path, os.O_RDWR)
+        self.os.truncate(fd, 10)
+        self.assertEqual(10, self.os.stat(file_path).st_size)
+        with self.open(file_path) as f:
+            self.assertEqual('0123456789', f.read())
+
+    def test_ftruncate(self):
+        if self.is_pypy:
+            # not correctly supported
+            self.skip_real_fs()
+        self.assert_raises_os_error(errno.EBADF, self.os.ftruncate, 50, 10)
+        file_path = self.make_path('some_file')
+        self.create_file(file_path, contents='0123456789012345')
+
+        fd = self.os.open(file_path, os.O_RDWR)
+        self.os.truncate(fd, 10)
+        self.assertEqual(10, self.os.stat(file_path).st_size)
+        with self.open(file_path) as f:
+            self.assertEqual('0123456789', f.read())
+
 
 class RealOsModuleTest(FakeOsModuleTest):
     def use_real_fs(self):

pyfakefs/tests/fake_pathlib_test.py

@@ -973,6 +973,13 @@ def test_utime(self):
         self.assertEqual(1, st.st_atime)
         self.assertEqual(2, st.st_mtime)
 
+    def test_truncate(self):
+        path = self.make_path('some_file')
+        self.create_file(path, contents='test_test')
+        self.os.truncate(self.path(path), length=4)
+        st = self.os.stat(path)
+        self.assertEqual(4, st.st_size)
+
 
 class RealPathlibUsageInOsFunctionsTest(FakePathlibUsageInOsFunctionsTest):
     def use_real_fs(self):