chat: added tags_frequency() function and test

tests/test_chat.py

@@ -14,7 +14,7 @@ class TestChat(CmmTestCase):
         self.chat = Chat([])
         self.message1 = Message(Question('Question 1'),
                                 Answer('Answer 1'),
-                                {Tag('atag1')},
+                                {Tag('atag1'), Tag('btag2')},
                                 file_path=pathlib.Path('0001.txt'))
         self.message2 = Message(Question('Question 2'),
                                 Answer('Answer 2'),
@@ -57,6 +57,11 @@ class TestChat(CmmTestCase):
         tags_cont = self.chat.tags(contain='2')
         self.assertSetEqual(tags_cont, {Tag('btag2')})
 
+    def test_tags_frequency(self) -> None:
+        self.chat.add_msgs([self.message1, self.message2])
+        tags_freq = self.chat.tags_frequency()
+        self.assertDictEqual(tags_freq, {'atag1': 1, 'btag2': 2})
+
     @patch('sys.stdout', new_callable=StringIO)
     def test_print(self, mock_stdout: StringIO) -> None:
         self.chat.add_msgs([self.message1, self.message2])
@@ -83,7 +88,7 @@ Answer 2
 Question 1
 {Answer.txt_header}
 Answer 1
-{TagLine.prefix} atag1
+{TagLine.prefix} atag1 btag2
 FILE: 0001.txt
 {'-'*terminal_width()}
 {Question.txt_header}