Better get_input_entity code flow

Plenty of unnecessary exceptions were being raised and caught when
the input parameters were already correct. Furthermore, since
8abc7ade22, the in-disk cache was no
longer being used (so using usernames always reached out to memory).

telethon/client/users.py

@@ -303,26 +303,35 @@ class UserMethods(TelegramBaseClient):
             If you need to get the ID of yourself, you should use
             `get_me` with ``input_peer=True``) instead.
         """
+        # Short-circuit if the input parameter directly maps to an InputPeer
+        try:
+            return utils.get_input_peer(peer)
+        except TypeError:
+            pass
+
+        # Next in priority is having a peer (or its ID) cached in-memory
+        try:
+            # 0x2d45687 == crc32(b'Peer')
+            if isinstance(peer, int) or peer.SUBCLASS_OF_ID == 0x2d45687:
+                return self._entity_cache[peer]
+        except (AttributeError, KeyError):
+            pass
+
+        # Then come known strings that take precedence
         if peer in ('me', 'self'):
             return types.InputPeerSelf()
 
+        # No InputPeer, cached peer, or known string. Fetch from disk cache
         try:
-            # First try to get the entity from cache, otherwise figure it out
-            return self._entity_cache[peer]
-        except KeyError:
+            return self.session.get_input_entity(peer)
+        except ValueError:
             pass
 
+        # Only network left to try
         if isinstance(peer, str):
             return utils.get_input_peer(
                 await self._get_entity_from_string(peer))
 
-        if not isinstance(peer, int) and (not isinstance(peer, TLObject)
-                                          or peer.SUBCLASS_OF_ID != 0x2d45687):
-            # Try casting the object into an input peer. Might TypeError.
-            # Don't do it if a not-found ID was given (instead ValueError).
-            # Also ignore Peer (0x2d45687 == crc32(b'Peer'))'s, lacking hash.
-            return utils.get_input_peer(peer)
-
         # If we're a bot and the user has messaged us privately users.getUsers
         # will work with access_hash = 0. Similar for channels.getChannels.
         # If we're not a bot but the user is in our contacts, it seems to work

telethon/entitycache.py

@@ -12,11 +12,12 @@ class EntityCache:
         Adds the given entities to the cache, if they weren't saved before.
         """
         if not utils.is_list_like(entities):
-            # Invariant: all "chats" and "users" are always iterables
+            # Invariant: all "chats" and "users" are always iterables,
+            # and "user" never is (so we wrap it inside a list).
             entities = itertools.chain(
-                [getattr(entities, 'user', None)],
                 getattr(entities, 'chats', []),
-                getattr(entities, 'users', [])
+                getattr(entities, 'users', []),
+                (hasattr(entities, 'user') and [entities.user]) or []
             )
 
         for entity in entities: