python enum fields with specified default value not serializable

In the generated Python code, default values for enums are set as enum objects (ie, <name: value>), but thrift objects expect this value to be stored either by its name or its value. This results in a keyerror during the write method. 

For example, if I have the following thrift schema

enum MyEnum {
  test_a,
  test_b,
  test_c
}
struct MyStruct {
  1: MyEnum enum_field = MyEnum.test_c
}
 

and then in the python code:

new_struct = MyStruct()
thrift.TSerialization.serialize(new_struct)

it'd get a `keyerror` from the code generated here 

oprot.writeI32(MyEnum[new_struct.enum_field].value)
  File "[my-path]/lib/python3.9/enum.py", line 432, in __getitem__
    return cls._member_map_[name]
KeyError: <MyEnum.test_c: 2>