escape '%' in specialized comparison explanations (fixes #63)

CHANGELOG

@@ -1,6 +1,7 @@
 Changes between 2.1.0 and 2.1.1.DEV
 ----------------------------------------------
 
+- fix assertion rewriting on inserts involving strings containing '%'
 - fix assertion rewriting on calls with a ** arg
 - don't cache rewritten modules if bytecode generation is disabled
 - fix assertion rewriting in read-only directories

_pytest/assertion/__init__.py

@@ -45,7 +45,13 @@ def pytest_configure(config):
                 config=config, op=op, left=left, right=right)
             for new_expl in hook_result:
                 if new_expl:
-                    return '\n~'.join(new_expl)
+                    res = '\n~'.join(new_expl)
+                    if mode == "rewrite":
+                        # The result will be fed back a python % formatting
+                        # operation, which will fail if there are extraneous
+                        # '%'s in the string. Escape them here.
+                        res = res.replace("%", "%%")
+                    return res
         m = monkeypatch()
         config._cleanup.append(m.undo)
         m.setattr(py.builtin.builtins, 'AssertionError',

testing/test_assertrewrite.py

@@ -275,6 +275,11 @@ class TestAssertionRewrite:
             assert myany(A() < 0)
         assert "<MY42 object> < 0" in getmsg(f)
 
+    def test_formatchar(self):
+        def f():
+            assert "%test" == "test"
+        assert getmsg(f).startswith("assert '%test' == 'test'")
+
 
 class TestRewriteOnImport: