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missing and default values should be specified in deserialized form #378

chadrik opened this Issue Jan 14, 2016 · 4 comments


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5 participants

chadrik commented Jan 14, 2016

Marshmallow expects missing and default values to be specified in a pre-serialized form, which is inconvenient and unintuitive.

Here is a schema which I would expect to work:

import datetime
import uuid

class TestSchema(Schema):
    id = fields.UUID(
    ts = fields.DateTime(

However, it does not:

schema = TestSchema()
({}, {'id': [u'Not a valid UUID.'], 'ts': [u'Not a valid datetime.']})

Instead, I have to convert my default and missing values to primitive types, which undermines the purpose of a library like marshmallow that already knows how to do this. For data types like datetimes, which have many possible serialized formats, I have to keep the formatting of my default and missing values in sync with my field properties, which is fragile and convoluted:

from email.Utils import formatdate

class TestSchema(Schema):
    id = fields.UUID(
        missing=lambda: str(uuid.uuid1()))
    ts = fields.DateTime(

In the case of default values, marshmallow does not attempt to validate them, so no error is raised, but the resulting document is not deserializable.


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immerrr commented Jan 20, 2016

I have hit this working with comma-delimited parameters in webargs.


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costrouc commented Jan 5, 2017

I had this exact same error. I find it unintuitive that does not work but instead requires missing=lambda: str(


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sloria commented Mar 21, 2017

I think the OP makes sense. I would gladly review and merge a PR for this.


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lafrech commented Mar 21, 2018

I just sent a PR to achieve this.

@chadrik, @costrouc, @sloria, feedback welcome.

lafrech added a commit to Nobatek/marshmallow that referenced this issue Mar 28, 2018

@sloria sloria closed this in #756 Mar 30, 2018

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