bpo-36871: Handle spec errors in assert_has_calls

Lib/unittest/mock.py

@@ -931,13 +931,21 @@ def assert_has_calls(self, calls, any_order=False):
         If `any_order` is True then the calls can be in any order, but
         they must all appear in `mock_calls`."""
         expected = [self._call_matcher(c) for c in calls]
-        cause = expected if isinstance(expected, Exception) else None
+        cause = next((e for e in expected if isinstance(e, Exception)), None)
         all_calls = _CallList(self._call_matcher(c) for c in self.mock_calls)
         if not any_order:
             if expected not in all_calls:
+                if cause is None:
+                    problem = 'Calls not found.'
+                else:
+                    problem = ('Error processing expected calls.\n'
+                               'Errors: {}').format(
+                                   [e if isinstance(e, Exception) else None
+                                    for e in expected])
                 raise AssertionError(
-                    'Calls not found.\nExpected: %r%s'
-                    % (_CallList(calls), self._calls_repr(prefix="Actual"))
+                    f'{problem}\n'
+                    f'Expected: {_CallList(calls)}\n'
+                    f'Actual: {self._calls_repr(prefix="Actual")}'
                 ) from cause
             return
 
@@ -2214,12 +2222,20 @@ def assert_has_awaits(self, calls, any_order=False):
         they must all appear in :attr:`await_args_list`.
         """
         expected = [self._call_matcher(c) for c in calls]
-        cause = expected if isinstance(expected, Exception) else None
+        cause = next((e for e in expected if isinstance(e, Exception)), None)
         all_awaits = _CallList(self._call_matcher(c) for c in self.await_args_list)
         if not any_order:
             if expected not in all_awaits:
+                if cause is None:
+                    problem = 'Awaits not found.'
+                else:
+                    problem = ('Error processing expected awaits.\n'
+                               'Errors: {}').format(
+                                   [e if isinstance(e, Exception) else None
+                                    for e in expected])
                 raise AssertionError(
-                    f'Awaits not found.\nExpected: {_CallList(calls)}\n'
+                    f'{problem}\n'
+                    f'Expected: {_CallList(calls)}\n'
                     f'Actual: {self.await_args_list}'
                 ) from cause
             return

Lib/unittest/test/testmock/testasync.py

@@ -1,5 +1,6 @@
 import asyncio
 import inspect
+import re
 import unittest
 
 from unittest.mock import (call, AsyncMock, patch, MagicMock, create_autospec,
@@ -632,3 +633,23 @@ def test_assert_not_awaited(self):
         asyncio.run(self._runnable_test())
         with self.assertRaises(AssertionError):
             self.mock.assert_not_awaited()
+
+    def test_assert_has_awaits_not_matching_spec_error(self):
+        async def f(): pass
+
+        mock = AsyncMock(spec=f)
+
+        with self.assertRaisesRegex(
+                AssertionError,
+                re.escape('Awaits not found.\nExpected:')) as cm:
+            mock.assert_has_awaits([call()])
+        self.assertIsNone(cm.exception.__cause__)
+
+        with self.assertRaisesRegex(
+                AssertionError,
+                re.escape('Error processing expected awaits.\n'
+                          "Errors: [None, TypeError('too many positional "
+                          "arguments')]\n"
+                          'Expected:')) as cm:
+            mock.assert_has_awaits([call(), call('wrong')])
+        self.assertIsInstance(cm.exception.__cause__, TypeError)

Lib/unittest/test/testmock/testmock.py

@@ -1426,6 +1426,25 @@ def f(a, b, c, d=None): pass
             mock.assert_has_calls(calls[:-1])
         mock.assert_has_calls(calls[:-1], any_order=True)
 
+    def test_assert_has_calls_not_matching_spec_error(self):
+        def f(): pass
+
+        mock = Mock(spec=f)
+
+        with self.assertRaisesRegex(
+                AssertionError,
+                re.escape('Calls not found.\nExpected:')) as cm:
+            mock.assert_has_calls([call()])
+        self.assertIsNone(cm.exception.__cause__)
+
+        with self.assertRaisesRegex(
+                AssertionError,
+                re.escape('Error processing expected calls.\n'
+                          "Errors: [None, TypeError('too many positional "
+                          "arguments')]\n"
+                          'Expected:')) as cm:
+            mock.assert_has_calls([call(), call('wrong')])
+        self.assertIsInstance(cm.exception.__cause__, TypeError)
 
     def test_assert_any_call(self):
         mock = Mock()

Misc/NEWS.d/next/Core and Builtins/2019-09-24-18-45-46.bpo-36871.p47knk.rst

@@ -0,0 +1,3 @@
+Improve error handling for the assert_has_calls and assert_has_awaits methods of
+mocks. Fixed a bug where any errors encountered while binding the expected calls
+to the mock's spec were silently swallowed, leading to misleading error output.