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I am trying to insert a collection of objects in a flask api. We use marshmallow for deserializing. My endpoint looks like this:

        @blp.arguments(SomeSchemas, location='json', as_kwargs=True)
        @blp.response(200, SomeSchemas)
        def post(self, some_schemas: SomeSchemas) -> dict:

The schema is a simple schema like this:

class SomeSchemas(ma.Schema):
    schemas = ma.fields.List(ma.fields.Nested(SomeSchema))

class SomeSchema(ma.Schema):
    a = ma.fields.String()
    b = ma.fields.Integer()

When i post to the endpoint, I do get a list of the correct data, but it comes in the form of dicts, instead of being correctly translated into the object.

I have also tried explicitly using a list of objects (List[SomeSchema], SomeSchema(many=True), etc.) but I can not seem to figure it out.

I assume this is a very common use case (providing a list of arguments) and that I am missing an obvious solution, but I can't seem to find any reference as to how to do this correctly. To be clear, I am looking for the correct way to call the endpoint with a list (or some other collection type, it does not matter) and have said list be correctly deserialized and with the correct object type.

Jérôme
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Oskar
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1 Answers1

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Disclaimer: flask-smorest maintainer speaking.

I don't think the issue is related to the fact that the input is a list.

IIUC, your problem is that you're gettings dicts, rather than objects, injected in the view function. This is the default marshmallow behaviour. It can be overridden in marshmallow by using a post_load hook to actually instantiate the object.

I generally don't do that. In practice I find it better to instantiate objects in the view function. For instance, in a PUT resource, I prefer to instantiate the existing item from DB then update it with new data. In this case it is better to have the new data as dict than object.

There may not be a single truth, here, it could be a matter of opinion, but while the idea of having the object instantiated in the decorator and passed to the view is appealing, it might be a bit of an abusive shortcut.

I realize this answer is of the "no idea but you shouldn't do it anyway" type. Just saying maybe you shouldn't struggle to achieve this.

This said, I'd be surprised if it worked with non-list / non-nested inputs and I'd wonder why it doesn't work specifically in this case.

Jérôme
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  • Thank you for your comment. I now realize you are completely right, I never instantiate the object even when using non list. What I actually do is create typed fields from the type, such as (from the example above): @blp.arguments(SomeSchema, location='json', as_kwargs=True) @blp.response(200, SomeSchema) def post(self, a: string, b: int) -> dict: In this way I can access and use the fields via their names. Do you have a tip on how to do this in a 'clean' way for a list of arguments? I am manually extracting fields now from the list of dicts - is that just how it should be done? – Oskar Apr 04 '22 at 11:19
  • Sorry for the bad formatting btw, I don't know how to properly format a comment. – Oskar Apr 04 '22 at 11:23
  • You can use backticks to mark code as such in comments. Longer pieces of code probably ough a question edit. – Jérôme Apr 04 '22 at 12:59
  • I generally use `as_kwargs=False` (the default) and get the payload as `args` in `def post(self, args)`, then instantiate my object with `Object(**args)` (for the simple general case where the input dict matches the object attributes). Have a look at the examples in flask-smorest docs (and code). – Jérôme Apr 04 '22 at 13:01
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    Okay thank you. The reason I use as_kwargs=True is that it is for work and I was asked to use that style from now on for 'code clarity', I did in fact do the exact same as you in the past. Anyways, thank you very much, super helpful! – Oskar Apr 04 '22 at 14:46
  • You should be able to do `Object(**kwargs)`. Unless other args from another stacked use_args call are mixed up. – Jérôme Apr 04 '22 at 14:50