load None value into special value, e.g. None -> []

[frogcjn]

I have many schemas which have some list fields, and I want all my list fields in my schemas have a feature, that is load a None into [] automatically. So when I operate with advanced class object, I don't need to worry about it is a None or a list.

This feature cannot be simply implemented by override _deserialize, because _deserialize will not be called if input value is None. (but _serialize will be called if attribute value is None, which is different behavior as _deserialize)

One way to implement this feature, is to add @post_load to each Schama, which needs lot of repeat code.

Another way to solve the problem, is to override deserialize in fields.List, which is not the recommended way to subclass Field

### Optional List & Set

# None, [] --load()-> []
# None, [] --dump()-> []

class SafeList(List):
    def _serialize(self, value, attr, obj):
        result = super(SafeList, self)._serialize(value, attr, obj)
        return [] if result is None else result

    def deserialize(self, value, attr=None, data=None): # or override _validate_missing, None -load-> []
        result = super(SafeList, self).deserialize(value, attr, data)
        return [] if result is None else result

[sloria]

Could you use a post_load method in a base class that your Schemas inherit from?

[sloria]

We do not plan to support this feature at this time. Closing for now.

[lafrech]

#1381 would allow a Field to interpret None as missing.

Combined with load_default, it would solve OP's case.