def __init__(self, field_name: str, field_type: type=None):
    self.field_name = field_name
    if callable(field_type):
        self.field_type = field_type()
    else:
        self.field_type = field_type
    if field_type:
        self.field_type.source = field_name
def __init__(self, field_name: str, field_type: type=None):
    self.field_name = field_name
    if callable(field_type):
        self.field_type = field_type()
    else:
        self.field_type = field_type
    if field_type:
        self.field_type.source = field_name