bpo-38669: patch.object now raises a helpful error (GH17510)

This means a clearer message is now shown when patch.object is called with two string arguments, rather than a class and a string argument.
(cherry picked from commit cd90a52)

Co-authored-by: Elena Oat <oat.elena@gmail.com>

Lib/unittest/mock.py

@@ -1587,6 +1587,10 @@ def _patch_object(
     When used as a class decorator `patch.object` honours `patch.TEST_PREFIX`
     for choosing which methods to wrap.
     """
+    if type(target) is str:
+        raise TypeError(
+            f"{target!r} must be the actual object to be patched, not a str"
+        )
     getter = lambda: target
     return _patch(
         getter, attribute, new, spec, create,

Lib/unittest/test/testmock/testpatch.py

@@ -105,6 +105,10 @@ def test():
         self.assertEqual(Something.attribute, sentinel.Original,
                          "patch not restored")
 
+    def test_patchobject_with_string_as_target(self):
+        msg = "'Something' must be the actual object to be patched, not a str"
+        with self.assertRaisesRegex(TypeError, msg):
+            patch.object('Something', 'do_something')
 
     def test_patchobject_with_none(self):
         class Something(object):

Misc/NEWS.d/next/Tests/2019-11-04-02-54-16.bpo-38669.pazXZ8.rst

@@ -0,0 +1 @@
+Raise :exc:`TypeError` when passing target as a string with :meth:`unittest.mock.patch.object`.