Fix accessing builtin dict's attrs for a missing field

[mahenzon]

I've faced this issue, when a schema has a field with name same as builtin dict's has attribute:

from marshmallow import fields, Schema


class MyNestedSchema(Schema):
    update = fields.Boolean(required=False)


class MySchema(Schema):
    data = fields.Nested(MyNestedSchema)


class MyObj:
    def __init__(self):
        self.data = {
            # NOT passing a value, so it's missing
            # 'update': True,
        }


def main():
    obj = MyObj()
    data = MySchema().dump(obj)
    print(data)


if __name__ == '__main__':
    main()

Fails with this:

Traceback (most recent call last):
  File "/home/khorenyan/.PyCharm2019.2/config/scratches/scratch_42.py", line 40, in <module>
    main()
  File "/home/khorenyan/.PyCharm2019.2/config/scratches/scratch_42.py", line 35, in main
    data = MySchema().dump(obj)
  File "/home/khorenyan/projs/marshmallow/src/marshmallow/schema.py", line 556, in dump
    result = self._serialize(processed_obj, many=many)
  File "/home/khorenyan/projs/marshmallow/src/marshmallow/schema.py", line 520, in _serialize
    value = field_obj.serialize(attr_name, obj, accessor=self.get_attribute)
  File "/home/khorenyan/projs/marshmallow/src/marshmallow/fields.py", line 311, in serialize
    return self._serialize(value, attr, obj, **kwargs)
  File "/home/khorenyan/projs/marshmallow/src/marshmallow/fields.py", line 566, in _serialize
    return schema.dump(nested_obj, many=many)
  File "/home/khorenyan/projs/marshmallow/src/marshmallow/schema.py", line 556, in dump
    result = self._serialize(processed_obj, many=many)
  File "/home/khorenyan/projs/marshmallow/src/marshmallow/schema.py", line 520, in _serialize
    value = field_obj.serialize(attr_name, obj, accessor=self.get_attribute)
  File "/home/khorenyan/projs/marshmallow/src/marshmallow/fields.py", line 311, in serialize
    return self._serialize(value, attr, obj, **kwargs)
  File "/home/khorenyan/projs/marshmallow/src/marshmallow/fields.py", line 1103, in _serialize
    elif value in self.truthy:
TypeError: unhashable type: 'dict'

It happens this way:

• When being serialized, object is passed to the marshmallow.utils._get_value_for_key
• because of the missing update field in data on the line obj[key] the KeyError exception is raised
• and on the next line the getattr(obj, key, default) is returned

If the key is not found here, it returns default, so if my field is 'update_' or 'qwerty', it works as expected -- the result is returned and it's {'data': {}}

But the 'update' matches the builtin dict's attribute (method) so it's returned. And that's not expected at all.

Of course, I could override the get_attribute method in MyNestedSchema this way:

class MyNestedSchema(Schema):
    def get_attribute(self, obj: dict, attr: str, default: Any):
        try:
            return obj[attr]
        except KeyError:
            return default

    update = fields.Boolean(required=False)

But I think, that needs to be fixed.

I thought about checking if the getattr(obj, key, default) expression returns a method of a builtin's object, or checking a key to be one of the builtin's attrs, for example:

>>> dir(dict)
['__class__', '__contains__', '__delattr__', '__delitem__', '__dir__', '__doc__', '__eq__', '__format__', '__ge__', '__getattribute__', '__getitem__', '__gt__', '__hash__', '__init__', '__init_subclass__', '__iter__', '__le__', '__len__', '__lt__', '__ne__', '__new__', '__reduce__', '__reduce_ex__', '__repr__', '__setattr__', '__setitem__', '__sizeof__', '__str__', '__subclasshook__', 'clear', 'copy', 'fromkeys', 'get', 'items', 'keys', 'pop', 'popitem', 'setdefault', 'update', 'values']

I'm not sure if that's a proper way to do this check. Maybe we need to check even for isinstance instead of the object's type, but here's my current solution.

[mahenzon]

@deckar01 any ideas? it's been a long time already. If we need any changes, let's discuss them. I'm not sure about explicit types to be passed to the schemas' methods