Fix/improve comparison of byte strings

Fixes #5260.

changelog/5260.bugfix.rst

@@ -0,0 +1 @@
+Improve/fix comparison of byte strings with Python 3.

src/_pytest/assertion/util.py

@@ -147,7 +147,9 @@ def assertrepr_compare(config, op, left, right):
     explanation = None
     try:
         if op == "==":
-            if istext(left) and istext(right):
+            if isinstance(left, bytes) and isinstance(right, bytes):
+                explanation = _diff_text(left, right, verbose)
+            elif istext(left) and istext(right):
                 explanation = _diff_text(left, right, verbose)
             else:
                 if issequence(left) and issequence(right):
@@ -199,11 +201,15 @@ def _diff_text(left, right, verbose=0):
 
     def escape_for_readable_diff(binary_text):
         """
-        Ensures that the internal string is always valid unicode, converting any bytes safely to valid unicode.
-        This is done using repr() which then needs post-processing to fix the encompassing quotes and un-escape
-        newlines and carriage returns (#429).
+        Ensure that the internal string is always valid unicode, converting
+        any bytes safely to valid unicode.
+        This is done using repr() which then needs post-processing to fix the
+        encompassing quotes and un-escape newlines and carriage returns (#429).
         """
-        r = six.text_type(repr(binary_text)[1:-1])
+        r = repr(binary_text)
+        if r.startswith("b'"):
+            r = r[1:]
+        r = six.text_type(r[1:-1])
         r = r.replace(r"\n", "\n")
         r = r.replace(r"\r", "\r")
         return r

testing/test_assertion.py

@@ -354,6 +354,13 @@ def test_multiline_text_diff(self):
         assert "- spam" in diff
         assert "+ eggs" in diff
 
+    def test_bytes_diff(self):
+        diff = callequal(b"spam", b"eggs")
+        if PY3:
+            assert diff == ["b'spam' == b'eggs'", "- spam", "+ eggs"]
+        else:
+            assert diff == ["'spam' == 'eggs'", "- spam", "+ eggs"]
+
     def test_list(self):
         expl = callequal([0, 1], [0, 2])
         assert len(expl) > 1