Add capsysbinary fixture

`capsysbinary` works like `capsys` but produces bytes for `readouterr()`.

_pytest/capture.py

@@ -180,7 +180,7 @@ class CaptureManager:
         item.add_report_section(when, "stderr", err)
 
 
-capture_fixtures = {'capfd', 'capfdbinary', 'capsys'}
+capture_fixtures = {'capfd', 'capfdbinary', 'capsys', 'capsysbinary'}
 
 
 def _ensure_only_one_capture_fixture(request, name):
@@ -207,6 +207,22 @@ def capsys(request):
         yield fixture
 
 
+@pytest.fixture
+def capsysbinary(request):
+    """Enable capturing of writes to sys.stdout/sys.stderr and make
+    captured output available via ``capsys.readouterr()`` method calls
+    which return a ``(out, err)`` tuple.  ``out`` and ``err`` will be ``bytes``
+    objects.
+    """
+    _ensure_only_one_capture_fixture(request, 'capsysbinary')
+    # Currently, the implementation uses the python3 specific `.buffer`
+    # property of CaptureIO.
+    if sys.version_info < (3,):
+        raise request.raiseerror('capsysbinary is only supported on python 3')
+    with _install_capture_fixture_on_item(request, SysCaptureBinary) as fixture:
+        yield fixture
+
+
 @pytest.fixture
 def capfd(request):
     """Enable capturing of writes to file descriptors 1 and 2 and make
@@ -506,10 +522,9 @@ class SysCapture:
         setattr(sys, self.name, self.tmpfile)
 
     def snap(self):
-        f = self.tmpfile
+        res = self.tmpfile.getvalue()
-        res = f.getvalue()
+        self.tmpfile.seek(0)
-        f.truncate(0)
+        self.tmpfile.truncate()
-        f.seek(0)
         return res
 
     def done(self):
@@ -528,6 +543,14 @@ class SysCapture:
         self._old.flush()
 
 
+class SysCaptureBinary(SysCapture):
+    def snap(self):
+        res = self.tmpfile.buffer.getvalue()
+        self.tmpfile.seek(0)
+        self.tmpfile.truncate()
+        return res
+
+
 class DontReadFromInput:
     """Temporary stub class.  Ideally when stdin is accessed, the
     capturing should be turned off, with possibly all data captured

changelog/2934.feature

@@ -0,0 +1,2 @@
+Add ``capsysbinary`` a version of ``capsys`` which returns bytes from
+``readouterr()``.

doc/en/capture.rst

@@ -85,9 +85,9 @@ of the failing function and hide the other one::
 Accessing captured output from a test function
 ---------------------------------------------------
 
-The ``capsys``, ``capfd``, and ``capfdbinary`` fixtures allow access to
+The ``capsys``, ``capsysbinary``, ``capfd``, and ``capfdbinary`` fixtures
-stdout/stderr output created during test execution.  Here is an example test
+allow access to stdout/stderr output created during test execution.  Here is
-function that performs some output related checks:
+an example test function that performs some output related checks:
 
 .. code-block:: python
 
@@ -115,11 +115,20 @@ same interface but allows to also capture output from
 libraries or subprocesses that directly write to operating
 system level output streams (FD1 and FD2).
 
+.. versionadded:: 3.3
+
+If the code under test writes non-textual data, you can capture this using
+the ``capsysbinary`` fixture which instead returns ``bytes`` from
+the ``readouterr`` method.  The ``capfsysbinary`` fixture is currently only
+available in python 3.
+
+
 .. versionadded:: 3.3
 
 If the code under test writes non-textual data, you can capture this using
 the ``capfdbinary`` fixture which instead returns ``bytes`` from
-the ``readouterr`` method.
+the ``readouterr`` method.  The ``capfdbinary`` fixture operates on the
+filedescriptor level.
 
 
 .. versionadded:: 3.0

testing/test_capture.py

@@ -470,6 +470,38 @@ class TestCaptureFixture(object):
         """)
         reprec.assertoutcome(passed=1)
 
+    @pytest.mark.skipif(
+        sys.version_info < (3,),
+        reason='only have capsysbinary in python 3',
+    )
+    def test_capsysbinary(self, testdir):
+        reprec = testdir.inline_runsource("""
+            def test_hello(capsysbinary):
+                import sys
+                # some likely un-decodable bytes
+                sys.stdout.buffer.write(b'\\xfe\\x98\\x20')
+                out, err = capsysbinary.readouterr()
+                assert out == b'\\xfe\\x98\\x20'
+                assert err == b''
+        """)
+        reprec.assertoutcome(passed=1)
+
+    @pytest.mark.skipif(
+        sys.version_info >= (3,),
+        reason='only have capsysbinary in python 3',
+    )
+    def test_capsysbinary_forbidden_in_python2(self, testdir):
+        testdir.makepyfile("""
+            def test_hello(capsysbinary):
+                pass
+        """)
+        result = testdir.runpytest()
+        result.stdout.fnmatch_lines([
+            "*test_hello*",
+            "*capsysbinary is only supported on python 3*",
+            "*1 error in*",
+        ])
+
     def test_partial_setup_failure(self, testdir):
         p = testdir.makepyfile("""
             def test_hello(capsys, missingarg):
@@ -1233,7 +1265,7 @@ def test_dontreadfrominput_has_encoding(testdir):
     reprec.assertoutcome(passed=1)
 
 
-def test_pickling_and_unpickling_enocded_file():
+def test_pickling_and_unpickling_encoded_file():
     # See https://bitbucket.org/pytest-dev/pytest/pull-request/194
     # pickle.loads() raises infinite recursion if
     # EncodedFile.__getattr__ is not implemented properly