Add readouterr_bytes for retrieving captured binary output

CHANGELOG.rst

@@ -16,6 +16,9 @@ New Features
 * ``pytest.raises`` now asserts that the error message matches a text or regex
   with the ``match`` keyword argument. Thanks `@Kriechi`_ for the PR.
 
+* ``capsys`` and ``capfd`` fixtures now expose a ``readouterr_bytes()`` method
+  which returns captured output as bytes.  Thanks `@asottile`_ for the PR.
+
 
 Changes
 -------

_pytest/capture.py

@@ -204,6 +204,12 @@ def readouterr(self):
         except AttributeError:
             return self._outerr
 
+    def readouterr_bytes(self):
+        try:
+            return self._capture.readouterr_bytes()
+        except AttributeError:
+            return self._outerr
+
     @contextlib.contextmanager
     def disabled(self):
         capmanager = self.request.config.pluginmanager.getplugin('capturemanager')
@@ -315,6 +321,12 @@ def readouterr(self):
         return (self.out.snap() if self.out is not None else "",
                 self.err.snap() if self.err is not None else "")
 
+    def readouterr_bytes(self):
+        """ return snapshot bytes value of stdout/stderr capturings. """
+        return (self.out.snap_bytes() if self.out is not None else b'',
+                self.err.snap_bytes() if self.err is not None else b'')
+
+
 class NoCapture(object):
     __init__ = start = done = suspend = resume = lambda *args: None
 
@@ -370,6 +382,14 @@ def snap(self):
             return res
         return ''
 
+    def snap_bytes(self):
+        f = self.tmpfile
+        f.seek(0)
+        res = f.read()
+        f.truncate(0)
+        f.seek(0)
+        return res
+
     def done(self):
         """ stop capturing, restore streams, return original capture file,
         seeked to position zero. """
@@ -416,6 +436,13 @@ def snap(self):
         f.seek(0)
         return res
 
+    def snap_bytes(self):
+        f = getattr(self.tmpfile, 'buffer', self.tmpfile)
+        res = f.getvalue()
+        f.truncate(0)
+        f.seek(0)
+        return res
+
     def done(self):
         setattr(sys, self.name, self._old)
         del self._old

testing/test_capture.py

@@ -898,6 +898,16 @@ def test_capturing_readouterr_decode_error_handling(self):
             out, err = cap.readouterr()
         assert out == py.builtin._totext('\ufffd\n', 'unicode-escape')
 
+    def test_capturing_bytes(self):
+        some_bytes = b'\xf1\xe7\x12\r\n'
+        with self.getcapture() as cap:
+            # Write bytes to output in a py2+py3
+            getattr(sys.stdout, 'buffer', sys.stdout).write(some_bytes)
+            getattr(sys.stderr, 'buffer', sys.stderr).write(some_bytes)
+            out, err = cap.readouterr_bytes()
+            assert out == some_bytes
+            assert err == some_bytes
+
     def test_reset_twice_error(self):
         with self.getcapture() as cap:
             print ("hello")