DictShield

Aside from being a cheeky excuse to make people say things that sound sorta dirty, DictShield is a database-agnostic modeling system. It provides a way to model, validate and reshape data easily. All without requiring any particular database.

A blog model might look like this:

from dictshield.document import Document
from dictshield.fields import StringField

class BlogPost(Document):
    title = StringField(max_length=40)
    body = StringField(max_length=4096)

DictShield objects serialize to JSON by default. Store them in Memcached, MongoDB, Riak, whatever you need.

Say we have some data coming in from an iPhone:

json_data = request.post.get('data')
data = json.loads(json_data)

Validating the data then looks like this: Model(**data).validate().

Easy.

The Design

DictShield aims to provides helpers for a few types of common needs for modeling. It has been useful on the server-side so far, but I believe it could also serve for building an RPC.

1. Creating Flexible Documents

2. Easy To Use With Databases Or Caches

3. A Type System

4. Validation Of Types

5. Input / Output Shaping

DictShield also allows for object hierarchy's to be mapped into dictionaries too. This is useful primarily to those who use DictShield to instantiate classes representing their data instead of just filtering dictionaries through the class's static methods.

Example Uses

There are a few ways to use DictShield. A simple case is to create a class structure that has typed fields. DictShield offers multiple types in fields.py, like an EmailField or DecimalField.

Creating Flexible Documents

Below is an example of a Media class with a single field, the title.

from dictshield.document import Document
from dictshield.fields import StringField

class Media(Document):
    """Simple document that has one StringField member
    """
    title = StringField(max_length=40)

You create the class just like you would any Python class. And we'll see how that class is represented as a Python dictionary.

m = Media()
m.title = 'Misc Media'
print 'From Media class as Python structure:\n\n    %s\n' % (m.to_python())

The output from this looks like:

{
    '_types': ['Media'],
    '_cls': 'Media',
    'title': u'Misc Media'
}

All the meta information is removed and we have just a barebones representation of our data. Notice that the class information is still there as _cls and _types.

More On Object Modeling

We see two keys that come from Media's meta class: _types and _cls. _types stores the hierachy of Document classes used to create the document. _cls stores the specific class instance. This becomes more obvious when I subclass Media to create the Movie document below.

import datetime
from dictshield.fields import IntField

class Movie(Media):
    """Subclass of Foo. Adds bar and limits publicly shareable
    fields to only 'bar'.
    """
    _public_fields = ['title','year']
    year = IntField(min_value=1950, 
                    max_value=datetime.datetime.now().year)
    personal_thoughts = StringField(max_length=255)

Here's an instance of the Movie class:

mv = Movie()
mv.title = u'Total Recall'
mv.year = 1990
mv.personal_thoughts = u'I wish I had three hands...'

This is the raw document as converted to a Python dictionary:

{
    'personal_thoughts': u'I wish I had three hands...', 
    '_types': ['Media', 'Media.Movie'], 
    'title': u'Total Recall', 
    '_cls': 'Media.Movie',
    'year': 1990
}

Notice that _types has kept track of the relationship between Movie and Media.

Upgrading Documents

Upgrading documents is then easy because you can add optional fields and remove them.

As instances are created, two things happen. The fields that don't belong are removed as data is sent back and forth between the client and the server. And fields that are new are allowed at the data layer, assuming the user experience layer will be catching up soon. Until then, the field can just be optional.

Easy To Use With Databases Or Caches

We could pass this directly to Mongo to save it.

>>> db.test_collection.save(m.to_python())

Or if we were using Riak.

>>> media = bucket.new('test_key', data=m.to_python())
>>> media.store()

Or maybe we're storing json in a memcached.

>>> mc["test_key"] = m.to_json()

A Type System

This is what the MD5Field looks like. Notice that it's basically just an implementation of a validate() function, which raises a DictPunch exception if validation fails.

class MD5Field(BaseField):
    """A field that validates input as resembling an MD5 hash.
    """
    hash_length = 32
    def validate(self, value):
        if len(value) != MD5Field.hash_length:
            raise DictPunch('MD5 value is wrong length',
                            self.field_name, value)
        try:
            x = int(value, 16)
        except:
            raise DictPunch('MD5 value is not hex',
                            self.field_name, value)

You might notice that the field which failed is also reported. It's available on the exception as field_name and field_value.

The exception prints in this pattern field_name(field_value): reason.

DictPunch caught: secret(whatevz):  MD5 value is wrong length

If you think the overhead of validation is unnecessary for some use cases, you can skip it by never calling validate().

Validation Of Types

As we saw above, we know we can validate Document instances by calling validate(). Let's generate a User instance with seed data and validate it.

First, here is the User model:

class User(Document):
    _public_fields = ['name']
    secret = MD5Field()
    name = StringField(required=True, max_length=50)
    bio = StringField(max_length=100)
    url = URLField()

Next, we seed the instance with some data and validate it.

user = User(**{'secret': 'whatevs', 'name': 'test hash'})
try:
    user.validate()
except DictPunch, dp:
    print 'DictPunch caught: %s' % (dp))

This calling validate() on a model validates an instance by looping through it's fields and calling field.validate() on each one.

We can still be leaner. DictShield also allows validating input without instantiating any objects.

Validating User Input

Let's say we get this JSON string from a user.

{"bio": "Python, Erlang and guitars!", "secret": "e8b5d682452313a6142c10b045a9a135", "name": "J2D2"}

We might write some server code that looks like this:

json_string = request.get_arg('data')
user_input = json.loads(json_string)
u.validate(**user_input)

This method builds a User instance out of the input, which also throws away keys that aren't in the User definition.

We then call validate() on that User instance to validate each field against what the dictionary contained. If the data doesn't pass exception, a DictPunch is thrown and we handle the error.

If validation passed, we're done. We know the data looks good.

Input / Output shaping

Input is coming from everyone online, so who knows what it's in there. We do, however, know exactly what fields we want to be there. Same goes for output.

A web system typically has tiers involved with data access, depending on the user logged in. My most common need is to differentiate between internal system data (the raw document), data fields for the owner of the data (internal data removed) and the data fields that are shareable with the general public.

Removing Unknown Fields

Unrecognized fields, in user input, are thrown away. This makes handling input fairly easy because you are generally working with a list of fields, what they look like and how to turn them into Python or JSON. Not much else.

So here's how you can reduce the user input into just the fields found on a User document.

Consider the following string:

{
    "rogue_field": "MWAHAHA", 
    "bio": "Python, Erlang and guitars!", 
    "secret": "e8b5d682452313a6142c10b045a9a135", 
    "name": "J2D2"
}

Parse it just like before.

user_doc = User(**total_input).to_python()

The values in total_input are matched against fields found in the DictShield Document class and everything else is discarded.

user_doc now looks like below with rogue_field removed.

{
    '_types': ['User'], 
    'bio': u'Python, Erlang and guitars!, 
    'secret': 'e8b5d682452313a6142c10b045a9a135', 
    'name': u'J2D2', 
    '_cls': 'User'
}

JSON for Owner of Document

Here is our Movie document safe for transmitting to the owner of the document. We achieve this by calling Movie.make_json_ownersafe. This function is a classmethod available on the Document class. It knows to remove _cls and _types because they are in Document._internal_fields. _You can add any fields that should be treated as internal to your system by adding a list named _private_fields to your Document and listing each field.

{
    "personal_thoughts": "I wish I had three hands...",
    "title": "Total Recall",
    "year": 1990
}

JSON for Public View of Document

This is dictionary safe for transmitting to the public, not just the owner. Get this by calling make_json_publicsafe.

{
    "title": "Total Recall",
    "year": 1990
}

Working Without Instances

Consider a user updating some of their settings. Rather than validate the entire document, you want to check validation for just the field the client is updating and tell your database to store just that field.

DictShield offers a few classmethods to facilitate this.

Class Level Validation

validate_class_fields gives us that by checking if some dictionary matches the pattern it needs, including required fields. Notice, it's also a classmethod. No need to instantiate anything.

user_input = {
    'url': 'http://j2labs.tumblr.com'
}

try:
    User.validate_class_fields(user_input)
except DictPunch, dp:
    print('  Validation failure: %s\n' % (dp))

This particular code would throw an exception because the name field is required, but not present.

validation_class_partial lets you validate only the fields present in the input. This is useful for updating one or two fields in a document at a time, like we attempted above.

...
    User.validate_class_partial(user_input)
...

Aggregating Errors

validate_class_fields also offers more full validation. Pass validate_all=True to return 0 or more exceptions. 0 exceptions indicates validation was successful.

exceptions = User.validate_class_fields(total_input, validate_all=True)

Installing

DictShield is in pypi so you can use easy_install or pip.

pip install dictshield

Contributors

• James Dennis
• Andrew Gwozdziewycz
• Dion Paragas
• Tom Waits
• Chris McCulloh

License

BSD!