boxes: Enable autocomplete for PM recipients.

Currently, the recipients box has no autocomplete for users names.
This commit adds a method to autocomplete names appended with their
corresponding emails and enables it. The typeahead autocompletes the
names with their emails, but only names are shown in the suggestions.

Tests added.

tests/ui_tools/test_boxes.py

@@ -286,6 +286,52 @@ def test_generic_autocomplete_emojis(self, write_box, text,
         typeahead_string = write_box.generic_autocomplete(text, state)
         assert typeahead_string == required_typeahead
 
+    @pytest.mark.parametrize(['text', 'matching_users',
+                              'matching_users_info'], [
+        ('', [
+            'Human Myself',
+            'Human 1',
+            'Human 2'
+        ], [
+            'Human Myself <[email protected]>',
+            'Human 1 <[email protected]>',
+            'Human 2 <[email protected]>'
+        ]),
+        ('My', ['Human Myself'],
+         ['Human Myself <[email protected]>']),
+    ], ids=[
+        'no_search_text',
+        'single_word_search_text',
+    ])
+    def test__to_box_autocomplete(self, mocker, write_box, text,
+                                  users_fixture, matching_users,
+                                  matching_users_info, state=1):
+        _process_typeaheads = mocker.patch(BOXES
+                                           + '.WriteBox._process_typeaheads')
+
+        write_box._to_box_autocomplete(text, state)
+
+        _process_typeaheads.assert_called_once_with(matching_users_info, state,
+                                                    matching_users)
+
+    @pytest.mark.parametrize('text, expected_text', [
+        ('Hu', 'Human Myself <[email protected]>'),
+        ('Human M', 'Human Myself <[email protected]>')
+    ])
+    def test__to_box_autocomplete_with_spaces(self, write_box, text,
+                                              expected_text, widget_size):
+        write_box.private_box_view(email='[email protected]',
+                                   recipient_user_ids=[1])
+        write_box.to_write_box.set_edit_text(text)
+        write_box.to_write_box.set_edit_pos(len(text))
+        write_box.focus_position = write_box.FOCUS_CONTAINER_HEADER
+        size = widget_size(write_box)
+
+        write_box.keypress(size, primary_key_for_command('AUTOCOMPLETE'))
+
+        assert (write_box.to_write_box.edit_text
+                == expected_text)
+
     @pytest.mark.parametrize(['text', 'state', 'to_pin', 'matching_streams'], [
         ('', 1, [], ['Secret stream', 'Some general stream',
                      'Stream 1', 'Stream 2']),

zulipterminal/ui_tools/boxes.py

@@ -80,6 +80,12 @@ def private_box_view(self, button: Any=None, email: str='',
 
         self.to_write_box = ReadlineEdit("To: ",
                                          edit_text=recipient_info)
+        self.to_write_box.enable_autocomplete(
+            func=self._to_box_autocomplete,
+            key=primary_key_for_command('AUTOCOMPLETE'),
+            key_reverse=primary_key_for_command('AUTOCOMPLETE_REVERSE')
+        )
+        self.to_write_box.set_completer_delims("")
 
         self.msg_write_box = ReadlineEdit(multiline=True)
         self.msg_write_box.enable_autocomplete(
@@ -178,6 +184,22 @@ def _set_stream_write_box_style(self, widget: ReadlineEdit,
         (self.header_write_box[self.FOCUS_HEADER_PREFIX_STREAM]
          .set_text((color, stream_marker)))
 
+    def _to_box_autocomplete(self, text: str, state: Optional[int]
+                             ) -> Optional[str]:
+        users_list = self.view.users
+
+        matching_users = [user
+                          for user in users_list
+                          if match_user(user, text)]
+
+        user_names_and_emails = ["{} <{}>".format(user['full_name'],
+                                 user['email']) for user in matching_users]
+
+        user_names = [user['full_name'] for user in matching_users]
+
+        return self._process_typeaheads(user_names_and_emails, state,
+                                        user_names)
+
     def _topic_box_autocomplete(self, text: str, state: Optional[int]
                                 ) -> Optional[str]:
         topic_names = self.model.topics_in_stream(self.stream_id)