Allow callables for fields.Nested

Suggested earlier here: https://github.com/marshmallow-code/marshmallow/issues/19#issuecomment-43685498.

The suggestion would be to allow:

foo = fields.Nested(lambda: Foo)

in addition to

foo = fields.Nested('Foo')

Seems a bit cleaner in general.

[sloria]

Reopening this because I think this would enable valid use cases that are currently unmet.

(1) Passing arbitrary arguments to the nested Schema (only many, exclude, and only are currently supported. This could solve https://github.com/marshmallow-code/marshmallow/issues/285#issuecomment-152577723.

# Pass arbitrary arguments
foo = fields.Nested(lambda: FooSchema(partial=True))

# https://github.com/marshmallow-code/marshmallow/issues/285#issuecomment-152577723
class TreeSchema(Schema):
    tree = fields.Nested(lambda: TreeSchema(many=True, strict=True))

(2) With a slight modification to the OP’s proposed API, users could easily implement a polymorphic nested field.

def make_schema(obj):
    if isinstance(obj, Foo):
        return FooSchema()
    elif isinstance(obj, Bar):
        return BarSchema()

class MySchema(Schema):
    poly = fields.Nested(make_schema)

Feedback welcome.

[deckar01]

Not in marshmallow currently, but it is a schema syntax I have been investigating. Also, https://github.com/justanr/marshmallow-annotations/issues/5.

Back on track: I don’t think Method and Function are viable substitutes for a complete Nested field. The suggested workaround is not something I would expect developers to discover on their own or something I would want to document. Strings work well and are easy to document/use for recursively schema references. If a callable is needed for more complex behavior, a custom field is probably the best tool for the job.