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Bug: Custom Method serialization of List items throws AttributeError #1993

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benedekh opened this issue May 29, 2022 · 2 comments · May be fixed by #1994
Open

Bug: Custom Method serialization of List items throws AttributeError #1993

benedekh opened this issue May 29, 2022 · 2 comments · May be fixed by #1994

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@benedekh
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Let's have the following Schema:

class ExampleSchema(Schema):
	dummy_list = fields.List(fields.Method(serialize="serialize_me"))

	def serialize_me(self, obj):
		return [str(value) for value in obj["dummy_list"]]

Let's have the following serialization code:

obj = {"dummy_list": ["hello", "world"]}
dumped = ExampleSchema().dump(obj)

ExampleSchema().dump(obj) throws AttributeError: 'List' object has no attribute 'serialize_me' error.

The full stack trace is:

    dumped = ExampleSchema().dump(obj)
..\src\marshmallow\schema.py:399: in __init__
    self._init_fields()
..\src\marshmallow\schema.py:975: in _init_fields
    self._bind_field(field_name, field_obj)
..\src\marshmallow\schema.py:1034: in _bind_field
    field_obj._bind_to_schema(field_name, self)
..\src\marshmallow\fields.py:757: in _bind_to_schema
    self.inner._bind_to_schema(field_name, self)
..\src\marshmallow\fields.py:1858: in _bind_to_schema
    getattr(schema, self.serialize_method_name)
E   AttributeError: 'List' object has no attribute 'serialize_me'

The bug exists in marshmallow 3.16.0.
benedekh added a commit to benedekh/marshmallow that referenced this issue May 29, 2022
@benedekh
marshmallow-code#1993 Fix serialization of List items if they are ser…
18603d5 
…ialized by fields.Method.
This was referenced May 29, 2022
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@lafrech
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lafrech commented Jun 26, 2022

I still need to get my head around this. I suppose Tuple would be impacted as well.

In your example, I don't understand why you use List in the first place. Shouldn't it be

class ExampleSchema(Schema):
	dummy_list = fields.Method(serialize="serialize_me")

	def serialize_me(self, obj):
		return [str(value) for value in obj["dummy_list"]]

?

@benedekh
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benedekh commented Jun 28, 2022 •
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@lafrech: That would be also a way to go. However, I wanted to keep the same data types for the field as in the data class.

Originally, I discovered the bug in a @dataclass, where I declared the fields type-correctly and then defined a Schema separately, because I had some special fields in the @dataclass that could not be (de)serialized normally. One of these fields was List[CustomDataType] (where CustomDataType is the type which had to be (de)serialized manually).

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Fix serialization of List items if they are serialized by fields.Method. benedekh/marshmallow
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@lafrech @benedekh