Why is it necessary to add missing=True when Required=False? Why is the error message "Field may not be null" instead of "Field may not be missing" ?

Assume the following schema:

from marshmallow import Schema, fields

class FooSchema(Schema):
    a = fields.Int(required=True)
    b = fields.Int(required=False)  # Forgot to include missing=None

This will not raise an error when the payload is missing the b field:

# No error, makes sense:
FooSchema().load({'a': 1})

However, it will raise an error when the payload contains a b field set to None:

# Raises ValidationError {'b': ['Field may not be null.']}
FooSchema().load({'a': 1, 'b': None})
# Why? What use case is this supposed to handle?
# Regardless, why is the  error "Field may not be null" instead of "Field may not be missing"?

The solution to this problem if to add missing=None:

class FooSchema(Schema):
    a = fields.Int(required=True)
    b = fields.Int(required=False, missing=None)

# No error
FooSchema().load({'a': 1})

# No error
FooSchema().load({'a': 1, 'b': None})

–

I have two questions about this behavior:

1. What use case is this behavior supposed to support? In what situation would I want an error to be raised if required=False but missing is not set to None?

I agree that if someone has this use case, the reasoning for this implementation is sensible in that this is the only non ambiguous way to support it.

That is understandable. But what I’m a bit confused about is the error message: Field may not be null.

1. Wouldn’t Field may not be missing make more sense? The field absolutely may be null- so long as the field is included in the payload.

[lafrech]

missing (now deprecated, replaced with load_default) is a default value to use when the payload doesn’t contain the field.

If required is False (which is the case by default), the field may be missing. But None is not accepted. It is not the same as a missing value. To allow None, you may set allow_none=True.

Also, setting load_default=None implies allow_none=True.

https://marshmallow.readthedocs.io/en/stable/marshmallow.fields.html#marshmallow.fields.Field

[mkmoisen]

@lafrech Thank you.