Fix for Python 3.8 and lower

Author: erezsh

runtype/dataclass.py

@@ -5,10 +5,11 @@
 import random
 from copy import copy
 import dataclasses
-from typing import Union, ForwardRef
+from typing import Union
 from abc import ABC, abstractmethod
 import inspect
 
+from .utils import ForwardRef
 from .common import CHECK_TYPES
 from .validation import TypeMismatchError, ensure_isa as default_ensure_isa
 from .pytypes import TypeCaster, type_caster, SumType, NoneType

runtype/pytypes.py

@@ -9,6 +9,7 @@
 import typing
 from datetime import datetime
 
+from .utils import ForwardRef
 from .base_types import DataType, Validator, TypeMismatchError
 from . import base_types
 from . import datetime_parse
@@ -340,7 +341,7 @@ def _to_canon(self, t):
         if isinstance(t, (base_types.Type, Validator)):
             return t
 
-        if isinstance(t, typing.ForwardRef):
+        if isinstance(t, ForwardRef):
             t = t._evaluate(self.frame.f_globals, self.frame.f_locals, set())
 
         if isinstance(t, tuple):

runtype/utils.py

@@ -1,4 +1,22 @@
 import inspect
+import sys
+
+if sys.version_info < (3, 7):
+    # python 3.6 
+    from typing import _ForwardRef as ForwardRef
+    _orig_eval = ForwardRef._eval_type
+elif sys.version_info < (3, 9):
+    from typing import ForwardRef
+    _orig_eval = ForwardRef._evaluate
+else:
+    from typing import ForwardRef
+
+if sys.version_info < (3, 9):
+    def _evaluate(self, g, l, _):
+        return _orig_eval(self, g, l)
+    ForwardRef._evaluate = _evaluate
+
+
 
 def get_func_signatures(typesystem, f):
     sig = inspect.signature(f)