Allow to open existing pipe file descriptor

- fixes #493

CHANGES.md

@@ -8,12 +8,14 @@ The released versions correspond to PyPi releases.
     (see [#549](../../issues/549))
 
 #### Fixes
+  * allow to open existing pipe file descriptor
+    (see [#493](../../issues/493))
   * do not truncate file on failed flush
-   (see [#548](../../issues/548))
+    (see [#548](../../issues/548))
   * suppress deprecation warnings while collecting modules
-   (see [#542](../../issues/542))
+    (see [#542](../../issues/542))
   * add support for `os.truncate` and `os.ftruncate`
-   (see [#545](../../issues/545))
+    (see [#545](../../issues/545))
 
 #### Infrastructure
   * fixed another problem with CI test scripts not always propagating errors

pyfakefs/fake_filesystem.py

@@ -4997,6 +4997,14 @@ def __init__(self, filesystem, fd):
         self.file_object = None
         self.filedes = None
 
+    def __enter__(self):
+        """To support usage of this fake pipe with the 'with' statement."""
+        return self
+
+    def __exit__(self, type, value, traceback):
+        """To support usage of this fake pipe with the 'with' statement."""
+        self.close()
+
     def get_object(self):
         return self.file_object
 
@@ -5008,6 +5016,10 @@ def read(self, numBytes):
         """Read from the real pipe."""
         return os.read(self.fd, numBytes)
 
+    def flush(self):
+        """Flush the real pipe?"""
+        pass
+
     def write(self, contents):
         """Write to the real pipe."""
         return os.write(self.fd, contents)
@@ -5084,6 +5096,12 @@ def call(self, file_, mode='r', buffering=-1, encoding=None,
 
         file_object, file_path, filedes, real_path = self._handle_file_arg(
             file_)
+        if file_object is None and file_path is None:
+            wrapper = FakePipeWrapper(self.filesystem, filedes)
+            file_des = self.filesystem._add_open_file(wrapper)
+            wrapper.filedes = file_des
+            return wrapper
+
         if not filedes:
             closefd = True
 
@@ -5170,6 +5188,8 @@ def _handle_file_arg(self, file_):
             # opening a file descriptor
             filedes = file_
             wrapper = self.filesystem.get_open_file(filedes)
+            if isinstance(wrapper, FakePipeWrapper):
+                return None, None, filedes, None
             self._delete_on_close = wrapper.delete_on_close
             file_object = self.filesystem.get_open_file(filedes).get_object()
             file_path = file_object.name

pyfakefs/tests/fake_os_test.py

@@ -2694,6 +2694,22 @@ def test_open_file_with_existing_pipe(self):
         self.os.close(write_fd)
         self.os.close(fd)
 
+    def test_read_write_pipe(self):
+        read_fd, write_fd = self.os.pipe()
+        self.assertEqual(4, self.os.write(write_fd, b'test'))
+        self.assertEqual(b'test', self.os.read(read_fd, 4))
+        self.os.close(read_fd)
+        self.os.close(write_fd)
+
+    def test_open_existing_pipe(self):
+        if self.is_pypy:
+            raise unittest.SkipTest('Does not work correctly with PyPy')
+        read_fd, write_fd = self.os.pipe()
+        with self.open(write_fd, 'wb') as f:
+            self.assertEqual(4, f.write(b'test'))
+        with self.open(read_fd, 'rb') as f:
+            self.assertEqual(b'test', f.read(4))
+
     def test_write_to_pipe(self):
         read_fd, write_fd = self.os.pipe()
         self.os.write(write_fd, b'test')