fix formatting utf-8 error explanation

CHANGELOG.rst

@@ -62,6 +62,9 @@
 * Fix (`#1290`_): support Python 3.5's `@` operator in assertion rewriting.
   Thanks `@Shinkenjoe`_ for report with test case and `@tomviner`_ for the PR.
 
+* Fix formatting utf-8 explanation messages (`#1379`_).
+
+.. _#1379: https://github.com/pytest-dev/pytest/issues/1379
 .. _#1366: https://github.com/pytest-dev/pytest/issues/1366
 .. _#1040: https://github.com/pytest-dev/pytest/pull/1040
 .. _#680: https://github.com/pytest-dev/pytest/issues/680

_pytest/assertion/util.py

@@ -18,6 +18,15 @@
 _reprcompare = None
 
 
+# the re-encoding is needed for python2 repr
+# with non-ascii characters (see issue 877 and 1379)
+def ecu(s):
+    try:
+        return u(s, 'utf-8', 'replace')
+    except TypeError:
+        return s
+
+
 def format_explanation(explanation):
     """This formats an explanation
 
@@ -28,6 +37,7 @@ def format_explanation(explanation):
     for when one explanation needs to span multiple lines, e.g. when
     displaying diffs.
     """
+    explanation = ecu(explanation)
     explanation = _collapse_false(explanation)
     lines = _split_explanation(explanation)
     result = _format_lines(lines)
@@ -131,14 +141,6 @@ def assertrepr_compare(config, op, left, right):
     left_repr = py.io.saferepr(left, maxsize=int(width/2))
     right_repr = py.io.saferepr(right, maxsize=width-len(left_repr))
 
-    # the re-encoding is needed for python2 repr
-    # with non-ascii characters (see issue 877)
-    def ecu(s):
-        try:
-            return u(s, 'utf-8', 'replace')
-        except TypeError:
-            return s
-
     summary = u('%s %s %s') % (ecu(left_repr), op, ecu(right_repr))
 
     issequence = lambda x: (isinstance(x, (list, tuple, Sequence)) and

testing/test_assertion.py

@@ -242,6 +242,9 @@ def __repr__(self):
         expl = callequal(A(), '1')
         assert expl
 
+    def test_format_nonascii_explanation(self):
+        assert util.format_explanation('λ')
+
     def test_mojibake(self):
         # issue 429
         left = 'e'