Allow Unknown values

Question

I am using colander while de-serializing JSON response. There is a field that is configured to have numbers only, but sometimes the response has null or non-numeric value for this field and then my application throws invalid error. Is there a way I could handle it in my program to give a default value, say NA, to the field. Here is an example to explain the situation:

This is a sample:

class SampleClass(colander.MappingSchema):
    name = colander.SchemaNode(colander.String())
    testMark  = colander.SchemaNode(colander.Float(), missing=None)


class Sample(colander.MappingSchema):
    type = colander.SchemaNode(colander.String())
    student = colander.SampleStudent()

I want the following JSON to be accepted and not throw error for testMark as null

{  
   "type":"test",
   "student":{  
      "name":"a name",
      "testMark":null
   }
}

My apologies if there is something wrong in code, this is my first time in python. Hope I have made the scenario clear. I would want to give the testMark field a default value, say NA, whenever the test mark is not a number.

PS This is just a sample scenario, can't post the actual scenario here.

Answer 1

You could write your own SchemaType that accepts any value but deserialises it to a float or null.

http://docs.pylonsproject.org/projects/colander/en/latest/extending.html

class MyFloat(colander.Float):

    def deserialize(self, node, cstruct):
        if isinstance(cstruct, float):
            return cstruct
        else:
            return null

I didn't test this, but I hope you get the idea.