The Open Event Server offers features for events with several tracks and venues. When we were designing the models for the API, we wanted to add default values for some fields in case they aren’t provided by the client. This blog discusses how we have implemented in the project using python decorators that complies to the DRY principle and is easy to test and maintain.

Problem

Let’s first discuss the problem at hand in detail. We use Marshmallow extensively in our project. Marshmallow is an ORM/ODM/framework-agnostic library for converting complex data types, such as objects, to and from native python data types. We use it for Validating the input data, Deserializing the input data to app-level objects and Serializing app-level objects to primitive Python types.

We can define Schema’s very easily using Marshmallow. It also provides an easy way to declare default values to the fields. Below is a sample schema declaration:

class SampleSchema(Schema):
    """
    Sample Schema declaration
    """

    class Meta:
        """
        Meta class for the Sample Schema
        """
        type_ = 'sample-schema'

    id = fields.Str(dump_only=True)
    field_without_default = fields.Str(required=True)
    field_with_default = fields.Boolean(required=True, default=False)

We have defined an id field for storing the unique ID of the Model. We have also defined two other fields. One of them named as “field_with_default” is a Boolean field and has a default value specified as False.

When a client makes a POST/PATCH request to the server, we first validate the input data sent to us by the clients against the defined schema. Marshmallow also supports schema validation but it doesn’t support using default values during deserialization of the input data. It meant that whenever the input data had a missing field, the server would throw a validation error even for a field for which the default values are defined. It was clearly wrong since if the default values are defined, we would want that value to be used for the field. This defeats the entire purpose of declaring default values at the first place.

So, we would ideally like the following behaviour from the Server:

1. If the values are defined in the input data, use it during validation.
2. If the value for a required field is not defined but default value has been defined in the Schema, then use that value.
3. If no value has been defined for a required field and it doesn’t have any default value specified, then throw an error.

Solution

Marshmallow provides decorators like @pre_load and @post_load for adding pre-processing and post-processing methods. We can use them to add a method in each of the Schema classes which takes in the input data and the schema and adds default values to fields before we validate the input.

The first approach which we took was to add the following method to each of the schema classes defined in the project.

@pre_load
def patch_defaults(schema, in_data):
        data = in_data.get('data')
        if data is None or data.get('attributes') is None:
            return in_data
        attributes = data.get('attributes')
        for name, field in schema.fields.items():
            dasherized_name = dasherize(name)
            attribute = attributes.get(dasherized_name)
            if attribute is None:
                attributes[dasherized_name] = field.default
        return in_data

The method loops over all the fields defined in the schema class using schema.fields.item(). dasherize is a helper function defined in the utils class which converts underscores(_) in the variable name to dashes(-). After replacing the underscores with dashes we check if the value for the attribute is None. If it is None, then we assign it the specified default value.

Enhancing the solution

The above solution works but there is a problem. We have around 50 schemas defined in the project. Copy pasting this method 50 times would definitely violate the DRY principle. Moreover if we need to change this method in the future, we would have to do it 50 times.

One way to avoid it would be to add the patch_defaults method in a separate file and add a helper method make_object in each of the schema classes which just calls it.

@pre_load
def make_object(self, in_data):
    return patch_defaults(self, in_data)

We would still be repeating the helper method in 50 different files but since it’s sole purpose is to call the patch_defaults method, we won’t have to make changes in 50 files.

It certainly works well but we can go a step further and make it even easier. We can define a class decorator which would add the above make_object method to the class.

def use_defaults():
    """
    Decorator added to model classes which have default values specified for one of it's fields
    Adds the make_object method defined above to the class.
    :return: wrapper
    """
    def wrapper(k, *args, **kwargs):
        setattr(k, "make_object", eval("make_object", *args, **kwargs))
        return k
    return wrapper

Now we can simply add the use_defaults() decorator on the schema class and it would work.

References

• Marshmallow: http://marshmallow.readthedocs.io/en/latest/index.html
• Flask: http://flask.pocoo.org