Follow PEP 484 - no implicit optionals

freqtrade/strategy/hyper.py

@@ -4,7 +4,7 @@ This module defines a base class for auto-hyperoptable strategies.
 """
 import logging
 from pathlib import Path
-from typing import Any, Dict, Iterator, List, Tuple, Type, Union
+from typing import Any, Dict, Iterator, List, Optional, Tuple, Type, Union
 
 from freqtrade.constants import Config
 from freqtrade.exceptions import OperationalException
@@ -36,7 +36,8 @@ class HyperStrategyMixin:
         self._ft_params_from_file = params
         # Init/loading of parameters is done as part of ft_bot_start().
 
-    def enumerate_parameters(self, category: str = None) -> Iterator[Tuple[str, BaseParameter]]:
+    def enumerate_parameters(
+            self, category: Optional[str] = None) -> Iterator[Tuple[str, BaseParameter]]:
         """
         Find all optimizable parameters and return (name, attr) iterator.
         :param category: