Issue #1680159: unicode coercion during an 'in' operation was masking

any errors that might occur during coercion of the left operand and
turning them into a TypeError with a message text that was confusing in
the given context.  This patch lets any errors through, as was already
done during coercion of the right hand side.

Lib/test/test_unicode.py

@@ -344,7 +344,8 @@ class UnicodeTest(
         # If the following fails either
         # the contains operator does not propagate UnicodeErrors or
         # someone has changed the default encoding
-        self.assertRaises(UnicodeError, 'g\xe2teau'.__contains__, u'\xe2')
+        self.assertRaises(UnicodeDecodeError, 'g\xe2teau'.__contains__, u'\xe2')
+        self.assertRaises(UnicodeDecodeError, u'g\xe2teau'.__contains__, '\xe2')
 
         self.assertTrue(u'' in '')
         self.assertTrue('' in u'')
@@ -375,6 +376,7 @@ class UnicodeTest(
         self.assertTrue(u'asdf' not in u'')
 
         self.assertRaises(TypeError, u"abc".__contains__)
+        self.assertRaises(TypeError, u"abc".__contains__, object())
 
     def test_formatting(self):
         string_tests.MixinStrUnicodeUserStringTest.test_formatting(self)

Misc/NEWS

@@ -12,6 +12,9 @@ What's New in Python 2.7 alpha 2?
 Core and Builtins
 -----------------
 
+- Issue #1680159: unicode coercion during an 'in' operation no longer masks
+  the underlying error when the coercion fails for the left hand operand.
+
 - Issue #7491: Metaclass's __cmp__ method was ignored.
 
 - Issue #7466: segmentation fault when the garbage collector is called

Objects/unicodeobject.c

@@ -6502,8 +6502,6 @@ int PyUnicode_Contains(PyObject *container,
     /* Coerce the two arguments */
     sub = PyUnicode_FromObject(element);
     if (!sub) {
-        PyErr_SetString(PyExc_TypeError,
-                        "'in <string>' requires string as left operand");
         return -1;
     }