The substitute for 'use=lambda' in new API

[imhoffd]

Just putting this here for others who may be wondering what to do after use= (the callable that let you preprocess the value, see this in pre 0.16.0 code).

For 0.16.0 onwards, use of marshmallow is encouraged. Marshmallow brings a whole slew of serialization/deserialization features, which is exactly what you're doing when you take web requests from various frameworks and turn them into native python data types: deserializing.

use= used to let you change the value of a field while it was parsed. You can accomplish that and more now with marshmallow's post_load decorator:

from marshmallow import Schema, post_load
from webargs import fields

class ArgSchema(Schema):
    name = fields.Str(required=True, location='querystring')

    @post_load
    def uppercase_name(self, data):
        data['name'] = data['name'].upper()
        return data

See the documentation for how to use marshmallow schemas in webargs.

Since you now have access to all fields in the data argument of your post_load method, you can do cross preprocessing, meaning you can change the value of one field based upon the value of another.

[sloria]

Thanks @dwieeb!

Another way to get the same functionality as use is through a custom field.

class Use(fields.Field):

    def __init__(self, func, child=None, **kwargs):
        super().__init__(**kwargs)
        self.func = func
        self.child = child

    def _deserialize(self, value, attr, data):
        if self.child:
            value = self.child.deserialize(value, attr, data)
        return self.func(value)

@use_args({
    'foo': Use(lambda v: v.upper(), fields.Str())
})
def myview(args):
    #...

[imhoffd]

Interesting. Perhaps this could exist in the webargs codebase as fields.Lambda or fields.Alter or something?

[sloria]

Perhaps in the future. In the interest of keeping the core small, I'm going to hold off on adding this to the code base for now.

[imhoffd]

👍 I agree.

Besides, I think I would actually prefer the marshmallow solution, as it allows access to other fields and does not require custom fields.

Thanks for webargs. With the marshmallow integration it's the best solution out there even without being "stable"... And I've used pretty much everything out there for flask, including rolling my own.

[sloria]

Thanks for the encouraging words!

[sloria]

I've moved this to the Wiki here: https://github.com/sloria/webargs/wiki/Recipes . Thanks again @dwieeb !

[imhoffd]

You didn't need to do that; I'm not sure if a migration guide of a lesser used feature from an alpha version is helpful to people, especially as the 1.0 code base matures.

What's more important is that the documentation somehow shows the pre-processing and post-processing of schemas. Currently, there's just a link to the marshmallow api in the "marshmallow integration" section. People coming into webargs with no prior marshmallow experience may be looking in the docs for how to process values before they enter their route functions. Right now, it looks as if webargs doesn't have this.

I could get around to writing up something for the docs if I find time.

[sloria]

I agree; I think it would be useful to have some documentation for pre- and post-processing behavior. I would certainly welcome a PR.