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I have just started using Pyramid for one of my projects and I have a case where in I need to validate a form field input, by taking that form field value and making a web-service call to assert the value's correctness. Like for example there is a field called your bank's CUSTOMER-ID. I need to take that(alone) as input and validate at the server level by making a web-service call (like http://someotherdomain/validate_customer_id/?customer_id=<input_value>)lets say.

I am using Colander for form schema management and Deform for all form validation logic. I am confused about where I need to place my validation logic for the CUSTOMER-ID case. Is it at MySchema().bind(customer_id=<input_value>) (which has a deferred validator that queries the web-service) or something at the form.validate(request.POST.items()) ? If I take the deferred validator's path, then MySchema().bind is raising colander.Invalid error for incorrect CUSTOMER-ID. Thats fine. But that error is not at the form level but at the schema level. So how would I tell the user about this in a sane way ?

I have good experience with Django forms so I was expecting something like clean method. A form error like form['customer_id'].error is what I am expecting at the template level. Is it possible with Pyramid's Deform or with Colander ?

Nanda Kishore
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2 Answers2

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So I think the big problem you're having is understanding the separation of concerns of Colander and Deform. Colander is what people like to call a general schema validation library. Which means we define a schema, where each node has a particular data type and some nodes might be required/optional. Colander is then able to validate that schema, and tell us whether or no the data we passed to colander conforms to that schema. As an example, in my web apps, I am often building apis that accept GET/POST params that need to be validated. So in Pyramid, let's say I have this scenario:

request.POST = {
    'post_id': 1,
    'author_id': 1,
    'unnecessary_attr': 'stuff'
}

I can then validate it like so:

# schema
schema = SchemaNode(Mapping(),
                    SchemaNode(Integer(), name='post_id'),
                    SchemaNode(Integer(), name='author_id'))
schema.deserialize(request.POST)

And it will error if it can't conform the data to the specified schema. So you can see, colander can actually be used to validate ANY set of data, whether that comes from POST/GET/JSON data. Deform on the other hand is a form library, and helps you create/validate forms. It uses colander for all of the validation needs and as you can see it pretty much just completely delegates validation to colander. So to answer your question, you would do all of your validation stuff in colander, and deform would mostly handle the rendering of your forms.

JayD3e
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    Thanks. Its lot clear now. But if the schema validation fails, how would I report that like a form error ? Like telling the user, "Invalid CUSTOMER_ID entered" ? Because they are at two different levels. – Nanda Kishore Jan 07 '14 at 09:00
  • @None-da in the colander ecosystem, the message you want to change is defined at the validator level, i.e. https://github.com/Pylons/colander/blob/master/colander/__init__.py#L314. Some of them accept a 'message' kwarg when you instantiate them, some do not. – JayD3e Jan 10 '14 at 02:07
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To see a vivid pyramid example application and deform in action look at todopyramid as a part of IndyPy Python Web Shootout. A todo application was implemented in pyramid, django, flask and bottle. I studied the pyramid example - it is well written, shows deform schema validation and uses bootstrap to show validation messages.

Find more pyramid tutorials here:

Sascha Gottfried
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