Zenn-La

A REST microframework for Google App Engine (webapp2 and ndb)

Contents

• Installation
• Example Usage
• Custom Serializers
• Validations
• ViewSets
• Routers
• Filtering
• Renderers

Installation:

Install using pip

$ pip install zennla

For help with adding third party Python packages in Google App Engine, read here

Example Usage:

Suppose you have a model named Pokemon:

from google.appengine.ext import ndb

class Pokemon(ndb.Model):
    name = ndb.StringProperty()
    type = ndb.StringProperty()
    number = ndb.IntegerProperty()

Define a serializer for this model:

from zennla.serializers import ModelSerializer

class PokemonSerializer(ModelSerializer):
    model = Pokemon

Define a viewset for this model:

from zennla.viewsets import ModelViewSet

class PokemonViewSet(ModelViewSet):
    serializer_class = PokemonSerializer
    model = Pokemon  # Optional - uses serializer_class's `model` instance by default

Add a route to your resource linking a base URL to the viewset:

import webapp2
from zennla.routers import route

app = webapp2.WSGIApplication([
    route('/pokemon', PokemonViewSet)
])

...and you have a working API for the Pokemon model! It supports GET and POST on pokemon/ and GET, PUT, DELETE on pokemon/<id>/ by default.

You can create a Pokemon object as:

{{base_url}}/pokemon/ [POST]
Body:
{
    "name": "Bulbasaur",
    "type": "Grass",
    "number": 1
}

Response (201):
{
    "name": "Bulbasaur",
    "type": "Grass",
    "number": 1,
    "id": 4785074604081152
}

You can fetch your Pokemon resource as:

{{base_url}}/pokemon/ [GET]

Response (200):
[
    {
        "name": "Bulbasaur",
        "type": "Grass",
        "number": 1,
        "id": 4785074604081152
    }
]

Or retrieve a specific Pokemon as:

{{base_url}}/pokemon/4785074604081152/ [GET]

Response (200):
{
    "name": "Bulbasaur",
    "type": "Grass",
    "number": 1,
    "id": 4785074604081152
}

Or update a Pokemon as:

{{base_url}}/pokemon/4785074604081152/ [PUT]
{
    "name": "Bulbasaur",
    "type": "Grass/Poison",
    "number": 1
}

Response (200):
{
    "name": "Bulbasaur",
    "type": "Grass/Poison",
    "number": 1,
    "id": 4785074604081152
}

Custom Serializers

You can set the following attributes in the ModelSerializer subclass

• include_fields: List of field names to be included in the serialized form of a model object.
• exclude_fields: List of field names to be excluded in the serialized form of a model object. Takes precedence over include_fields
• translate_fields: A dict mapping field names to the names used in serialized representation

You can override to_dict_repr method to define your own dictionary representation of a model object.

You can also add a method named as get_<field_name> to define a custom representation for that field.

You can add pre and post save hooks (methods that run just before and after an object is written to the datastore respectively) by defining pre_save(self, instance, data, validated_data) and post_save(self, instance, data, validated_data) respectively.

Example:

class PokemonSerializer(ModelSerializer):
    model = Pokemon
    include_fields = ["name", "number"]  # Only include these fields
    translate_fields = {
        "number": "code"  # Translate `number` to `code` in serialized representation
    }

    def get_name(obj):
        return "Pokemon " + obj.name

{{base_url}}/pokemon/ [GET]
[
    {
        "name": "Pokemon Bulbasaur",
        "code": 1
    }
]

Validations

The ModelSerializer by default performs a validation for field type on the input data. In addition to that, you can add your own field validations. You can also perform object level validations. The field validations should be methods in your ModelSerializer named as validate_<field_name>. The method should raise a ValidationError if the input field value is not valid, or return silently. You can define any object level validations by defining the validate(self, data, model) method.

The validation flow is:

• basic validations
• field validations
• object level validation

Example:

from zennla.exceptions import ValidationError

class PokemonSerializer(ModelSerializer):
    model = Pokemon

    def validate_name(self, field_value):
        """
        Only accept lower-case names
        """
        if field_value != field_value.lower():
            raise ValidationError("Name must be lower case!")

{{base_url}}/pokemon/ [POST]
{
    "name": "Mewtwo",
    "type": "Psychic"
    "number": 150
}

Response (400):
{
    "detail": "Name must be lower case!"
}

Viewsets

Viewsets are request handler classes which provide CRUD operations. To define custom viewsets, you can override get(), put(), post() and delete() or any other method corresponding to the allowed methods.

Customizing

• You can also add pre and post method handler hooks to perform any actions. These should be defined as pre_<handler_method> or post_<handler_method>.

• You can add filtering to the viewsets by listing the FilterSets in the filter_backends attribute (Discussed in detail here).

• You can support a number of media types by listing the renderers in the renderers attribute (Discussed in detail here)

• Overriding get_query(): You can override get_query() to perform any filtering of the result set before serialization.

• Overriding get_serializer_class(): You can override get_serializer_class() to choose a serializer class dynamically.

• Overridding get_model(): You can override get_model() to choose a model dynamically. Defaults to the model defined by the model attribute or, if not defined, the model attribute of the serializer_class.

Example

class PokemonViewSet(ModelViewSet):
    serializer_class = PokemonSerializer

    def pre_get(self, *args, **kwargs):
        # perform_some_action
        return

    def get_query():
        """
        Only query on Grass type Pokemon
        """
        return Pokemon.query(Pokemon.type == 'grass')

Routers

Routers provide routing mechanism for resources. It contains a route() function which returns a routes.PathPrefixRoute (read here) mapping a request handler viewset with a base_url.

The viewset must:

• have get(), post(), put(), delete() defined or
• extend viewset.ModelViewSet

The routes.PathPrefixRoute has two routes:

• The list view at base_url/ with URI: base_url-list
• The detail view at base_url/ with URL base_url-detail

Optional Parameters:

• detail_field: The name of the field used to identify a resource
• allowed_list_methods: List of HTTP methods allowed for list view. Default is [GET, POST]
• allowed_detail_methods: List of HTTP methods allowed for detail view. Default is [GET, PUT, DELETE]

Filtering

You can create filters by extending the FilterSet class. The FilterSet class lists all the filters that need to be applied to a query and contains a method get_filtered_query() which filters the query using the listed filters. The FilterSet must have an attribute whose name becomes a query parameter and value is equal to one of the FieldFilters. The FieldFilters defined by default are:

• NumberFilter: Use this to make a filter that takes numeric values
• BooleanFilter: Use this to make a filter that takes boolean values
• StringFilter: Use this to make a filter that takes string values

A FilterField needs to be specified an ndb model field on which it should be applied. It also has a lookup_type attribute which can be used to define the type of lookup. The valid values are:

• 'eq': Check for equality (default)
• 'in': Compare against a list of values
• 'ne': Check for inequality
• 'le': Check for less than or equals (<=)
• 'ge': Check for greater than or equals (>=)
• 'lt': Check for less than (<)
• 'gt': Check for greater than (>)

You can create your own FilterFields by overriding get_converted_value() which takes in a raw value (string) and converts it into the format required before any comparisons are done.

Example

from zennla import filters

class PokemonFilter(filters.FilterSet):
    name = filters.StringFilter(Pokemon.name)
    type = filters.StringFilter(Pokemon.type, lookup_type='in')
    num_ge = filters.NumberFilter(Pokemon.number, lookup_type='ge')

class PokemonViewSet(ModelViewSet):
    serializer_class = PokemonSerializer
    filter_backends = [PokemonFilter]  # Add the filterset to our viewset

{{base_url}}/pokemon/?name='Mewtwo'&type='Grass'&type='Psychic'&type='Electric'&num_ge=75 [GET]
{
    "name": "Mewtwo",
    "type": "Psychic"
    "number": 150
}

Response (200):
[
    {
        "name": "Mewtwo",
        "type": "Psychic"
        "number": 150
    }
]

Renderers

Renderers are used to serialize a response into specific media types. They give a generic way of being able to handle various media types on the response, such as JSON encoded data or HTML output.

By default Zenn-La provides a JSONRenderer and an XMLRenderer. You can define custom renderers by subclassing BaseRenderer and setting the media_type and format attributes, and overriding the render() method.

The renderer is chosen corresponding to the Accept header set on the request (read here). If no satisfying renderer is found associated with the viewset, a 406 - Not Acceptable response is returned. If no Accept header is set, the first renderer in the renderers list of the view set is used. By default, ModelViewSet uses JSONRenderer.