refactor: boxes: Simplify generic_autocomplete using autocomplete_forms.

This introduces `autocomplete_forms` to simplify the if-elif-else block
in `generic_autocomplete`.

Author: preetmishra

zulipterminal/ui_tools/boxes.py

@@ -96,16 +96,22 @@ def stream_box_view(self, caption: str='', title: str='') -> None:
         self.contents = write_box
 
     def generic_autocomplete(self, text: str, state: int) -> Optional[str]:
+        autocomplete_forms = {
+            'emojis': self.autocomplete_emojis,
+            'mentions': self.autocomplete_mentions,
+            'streams': self.autocomplete_streams,
+        }
         if text.startswith('@_'):
-            typeahead = self.autocomplete_mentions(text, '@_')
+            form, prefix_string = 'mentions', '@_'
         elif text.startswith('@'):
-            typeahead = self.autocomplete_mentions(text, '@')
+            form, prefix_string = 'mentions', '@'
         elif text.startswith('#'):
-            typeahead = self.autocomplete_streams(text, '#')
+            form, prefix_string = 'streams', '#'
         elif text.startswith(':'):
-            typeahead = self.autocomplete_emojis(text, ':')
+            form, prefix_string = 'emojis', ':'
         else:
             return text
+        typeahead = autocomplete_forms[form](text, prefix_string)
 
         try:
             return typeahead[state]