bpo-36871: Ensure method signature is used when asserting mock calls to a method (GH13261)

* Fix call_matcher for mock when using methods

* Add NEWS entry

* Use None check and convert doctest to unittest

* Use better name for mock in tests. Handle _SpecState when the attribute was not accessed and add tests.

* Use reset_mock instead of reinitialization. Change inner class constructor signature for check

* Reword comment regarding call object lookup logic

Lib/unittest/mock.py

@@ -804,6 +804,35 @@ def _format_mock_failure_message(self, args, kwargs, action='call'):
         return message % (action, expected_string, actual_string)
 
 
+    def _get_call_signature_from_name(self, name):
+        """
+        * If call objects are asserted against a method/function like obj.meth1
+        then there could be no name for the call object to lookup. Hence just
+        return the spec_signature of the method/function being asserted against.
+        * If the name is not empty then remove () and split by '.' to get
+        list of names to iterate through the children until a potential
+        match is found. A child mock is created only during attribute access
+        so if we get a _SpecState then no attributes of the spec were accessed
+        and can be safely exited.
+        """
+        if not name:
+            return self._spec_signature
+
+        sig = None
+        names = name.replace('()', '').split('.')
+        children = self._mock_children
+
+        for name in names:
+            child = children.get(name)
+            if child is None or isinstance(child, _SpecState):
+                break
+            else:
+                children = child._mock_children
+                sig = child._spec_signature
+
+        return sig
+
+
     def _call_matcher(self, _call):
         """
         Given a call (or simply an (args, kwargs) tuple), return a
@@ -811,7 +840,12 @@ def _call_matcher(self, _call):
         This is a best effort method which relies on the spec's signature,
         if available, or falls back on the arguments themselves.
         """
-        sig = self._spec_signature
+
+        if isinstance(_call, tuple) and len(_call) > 2:
+            sig = self._get_call_signature_from_name(_call[0])
+        else:
+            sig = self._spec_signature
+
         if sig is not None:
             if len(_call) == 2:
                 name = ''