Adds diff for differing strings.

expecter.py

@@ -1,4 +1,5 @@
 __all__ = ['expect']
+import difflib
 
 
 class expect(object):
@@ -35,8 +36,13 @@ def __getattr__(self, name):
             return getattr(super(expect, self), name)
 
     def __eq__(self, other):
-        assert self._actual == other, (
-            'Expected %s but got %s' % (repr(other), repr(self._actual)))
+        msg = 'Expected %s but got %s' % (repr(other), repr(self._actual))
+        if isinstance(other, str) and isinstance(self._actual, str):
+            diff = difflib.unified_diff(other.split('\n'),
+                    self._actual.split('\n'),
+                    lineterm='')
+            msg = '\n'.join([msg, 'Diff:'] + list(diff)[2:])
+        assert self._actual == other, msg
         return self
 
     def __ne__(self, other):

tests/unit/test_expecter.py

@@ -12,6 +12,17 @@ def _fails(): expect(1) == 2
         assert_raises(AssertionError, _fails)
         assert fail_msg(_fails) == 'Expected 2 but got 1'
 
+    def it_shows_diff_when_strings_differ(self):
+        def _fails(): expect('foo\nbar') == 'foo\nbaz'
+        assert_raises(AssertionError, _fails)
+        assert fail_msg(_fails) == ("Expected 'foo\\nbaz' but got 'foo\\nbar'\n"
+               "Diff:\n"
+               "@@ -1,2 +1,2 @@\n"
+               " foo\n"
+               "-baz\n"
+               "+bar"
+               ), repr(fail_msg(_fails))
+
     def it_expects_not_equals(self):
         expect(1) != 2
         def _fails(): expect(1) != 1