reencode non-ascii python2 assertion reprs, fixes #877

i decided against using a warning since the problem goes away with python3
the support code can be removed once we drop python2 in 10 years or so

Author: RonnyPfannschmidt

Diff for: CHANGELOG

@@ -193,6 +193,9 @@
 2.7.3 (compared to 2.7.2)
 -----------------------------
 
+- fix issue 877: propperly handle assertion explanations with non-ascii repr
+  Thanks Mathieu Agopian for the report
+
 - Allow 'dev', 'rc', or other non-integer version strings in `importorskip`.
   Thanks to Eric Hunsberger for the PR.
 

Diff for: _pytest/assertion/util.py

@@ -129,7 +129,14 @@ def assertrepr_compare(config, op, left, right):
     width = 80 - 15 - len(op) - 2  # 15 chars indentation, 1 space around op
     left_repr = py.io.saferepr(left, maxsize=int(width/2))
     right_repr = py.io.saferepr(right, maxsize=width-len(left_repr))
-    summary = u('%s %s %s') % (left_repr, op, right_repr)
+
+    # the reencoding is needed for python2 repr
+    # with non-ascii characters (see isssue 877)
+    summary = u('%s %s %s') % (
+        u(left_repr, 'utf-8', 'replace'),
+        op,
+        u(right_repr, 'utf-8', 'replace'),
+    )
 
     issequence = lambda x: (isinstance(x, (list, tuple, Sequence))
                             and not isinstance(x, basestring))

Diff for: testing/test_assertion.py

@@ -231,6 +231,17 @@ def test_unicode(self):
         assert expl[1] == py.builtin._totext('- £€', 'utf-8')
         assert expl[2] == py.builtin._totext('+ £', 'utf-8')
 
+    def test_nonascii_text(self):
+        """
+        :issue: 877
+        non ascii python2 str caused a UnicodeDecodeError
+        """
+        class A(str):
+            def __repr__(self):
+                return '\xff'
+        expl = callequal(A(), '1')
+        assert expl
+
     def test_mojibake(self):
         # issue 429
         left = 'e'