Small worked example

small_example.py

from dataclasses import dataclass
from dataclasses_json import dataclass_json
from datetime import datetime
from typing import Optional

@dataclass_json
@dataclass
class FirstExample:
    some_field: str
    some_date: datetime
    

right_now = datetime.now()

a = FirstExample(some_field="Hello", some_date=right_now)
# this works as expected:
print(FirstExample.schema().dumps(a))

# now, let's make one with an optional and default argument:

@dataclass_json
@dataclass
class SecondExample:
    some_field: str
    first_date: datetime
    second_date: Optional[datetime] = None
    

b = SecondExample(some_field="Hello", first_date=right_now, second_date=None)

# this will work:
print(b.to_json())

# this will not:
print(SecondExample.schema().dumps(b))

# > python small_example.py
# {"some_date": 1569545970.938367, "some_field": "Hello"}
# {"some_field": "Hello", "first_date": 1569545970.938367, "second_date": null}
# Traceback (most recent call last):
#   File "small_example.py", line 35, in <module>
#     print(SecondExample.schema().dumps(b))
#   File "/Users/bedricks/.pyenv/versions/3.7.2/lib/python3.7/site-packages/dataclasses_json/mm.py", line 292, in dumps
#     return Schema.dumps(self, *args, **kwargs)
#   File "/Users/bedricks/.pyenv/versions/3.7.2/lib/python3.7/site-packages/marshmallow/schema.py", line 583, in dumps
#     serialized = self.dump(obj, many=many)
#   File "/Users/bedricks/.pyenv/versions/3.7.2/lib/python3.7/site-packages/marshmallow/schema.py", line 553, in dump
#     result = self._serialize(processed_obj, many=many)
#   File "/Users/bedricks/.pyenv/versions/3.7.2/lib/python3.7/site-packages/marshmallow/schema.py", line 517, in _serialize
#     value = field_obj.serialize(attr_name, obj, accessor=self.get_attribute)
#   File "/Users/bedricks/.pyenv/versions/3.7.2/lib/python3.7/site-packages/marshmallow/fields.py", line 325, in serialize
#     return self._serialize(value, attr, obj, **kwargs)
#   File "/Users/bedricks/.pyenv/versions/3.7.2/lib/python3.7/site-packages/dataclasses_json/mm.py", line 26, in _serialize
#     return value.timestamp()
# AttributeError: 'NoneType' object has no attribute 'timestamp'
# >