Update required field status

[morgan]

Pretty normal operation, feel like I am possibly missing something. Have a schema for creation where fields are required in order to create the object. Then during edit (such as a PATCH operation), certain fields are no longer required. Such as updating a user that required a password at creation but doesn't require the password field for an update.

Currently, I'm doing something like this to solve the problem:

fields = ('first_name', 'last_name')
if 'password' in input:
    fields += ('password',)

result, errors = UserSchema(only=fields).load(input)

Feel there should be a more elegant way to modify the field itself. What are your recommendations for handling this?

[sloria]

@morgan If you're using a framework with a thread-local request object, you could do something like:

from flask import request
from marshmallow import missing, Schema, fields, ValidationError

def required_on_post(val):
    if request.method == 'POST':
        if val is missing:
            raise ValidationError('Field is required')

class UserSchema(Schema):
    password = fields.Str(validate=required_on_post, load_only=True)

Of course, this can be generalized with a validator class like RequiredForMethods.

[sloria]

Also see #272 for a similar question and solution.

Closing this for now.

[gw]

Hey @sloria ! Grateful Marshmallow user here--just wanted to update this issue as I recently came across it in Google search results.

This particular solution will not work in 2.7.3 because Field instance validators (i.e. those passed via the validate kwarg to a Field instance) will not get called if the field is not represented in the data to load.

To the other forsaken souls finding this issue: use partial (PR, docs).