Fix SystemError when using unicode_escape on Python 3

Fix #1087

_pytest/python.py

@@ -1053,10 +1053,14 @@ if _PY3:
            want to return escaped bytes for any byte, even if they match
            a utf-8 string.
         """
-        # source: http://goo.gl/bGsnwC
-        import codecs
-        encoded_bytes, _ = codecs.escape_encode(val)
-        return encoded_bytes.decode('ascii')
+        if val:
+            # source: http://goo.gl/bGsnwC
+            import codecs
+            encoded_bytes, _ = codecs.escape_encode(val)
+            return encoded_bytes.decode('ascii')
+        else:
+            # empty bytes crashes codecs.escape_encode (#1087)
+            return ''
 else:
     def _escape_bytes(val):
         """
@@ -1064,7 +1068,7 @@ else:
         is a full ascii string, otherwise escape it into its binary form.
         """
         try:
-            return val.encode('ascii')
+            return val.decode('ascii')
         except UnicodeDecodeError:
             return val.encode('string-escape')
 

testing/python/metafunc.py

@@ -137,6 +137,23 @@ class TestMetafunc:
         for val, expected in values:
             assert _idval(val, 'a', 6, None) == expected
 
+    def test_bytes_idval(self):
+        """unittest for the expected behavior to obtain ids for parametrized
+        bytes values:
+        - python2: non-ascii strings are considered bytes and formatted using
+        "binary escape", where any byte < 127 is escaped into its hex form.
+        - python3: bytes objects are always escaped using "binary escape".
+        """
+        from _pytest.python import _idval
+        values = [
+            (b'', ''),
+            (b'\xc3\xb4\xff\xe4', '\\xc3\\xb4\\xff\\xe4'),
+            (b'ascii', 'ascii'),
+            (u'αρά'.encode('utf-8'), '\\xce\\xb1\\xcf\\x81\\xce\\xac'),
+        ]
+        for val, expected in values:
+            assert _idval(val, 'a', 6, None) == expected
+
     @pytest.mark.issue250
     def test_idmaker_autoname(self):
         from _pytest.python import idmaker