Figuring out the correct field type from an untagged dump is a matter of guesswork and can be error-prone. A more explicit approach like https://github.com/Bachmann1234/marshmallow-polyfield/ is better. This project remains here to avoid breaking existing software.



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Union fields for marshmallow.

  • Free software: MIT license


This library works by trying a list of fields one by one, and (de)serializing with the first one not to raise an error. The type of the values is not taken into account, so if one of the fields in the union accepts values of an unexpected type, they will be used for serialization. This can lead to a surprising behavior, because :

u = Union(fields=[fields.Integer(), fields.String()]) # the Integer field accepts string representations of integers
type(u.deserialize('0')) # -> int

If you want to have precise control of which field will be used for which value, you can use marshmallow-polyfield instead of this library.