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Data transformations for dictshield documents
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README.md

README.md

dictshield-views

dictshield-views provides data transformations on DictShield objects.

TL;DR (Show me the code!)

    from dictshield.documents import Document
    from dictshield.views import View

    class Public(WhitelistView):
     fields = ['name', 'body']

    class Post(Document):
     name = StringField()
     body = StringField()
     username = StringField()
     password = PasswordField()

    >>> entry = Post(**{'name':'1', 'body':'2', 'username':'3', 'password':'4'})
    >>> entry.name
    '1'
    >>> entry.password
    '4'
    >>> Public(entry).name
    '1'
    >>> Public(entry).password
    AttributeError: 'Public' doesn't contain the attribute 'entry'

Definitions

Background

The idea for dictshield-views came from a discussion on brubeck-dev. James Dennis was thinking of how to represent permissioning on a per-field basis for DictShield Documents. The concept of whitelisting and blacklisting fields came up, which reminded me of data transformations provided by GWT's RequestFactory proxies. With these proxies, you can transform your server-side representation of an object to a smaller view (akin to database views) in order to minimize transportation costs.

Overview

dictshield-views provides View objects, which allow you to customize the representation of the original object. Views can be though of in two major groups: restrictive views or transformational views.

A great example of a restrictive view is a field white or black list. Given the object:

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

    class User(Document):
      username = StringField();
      full_name = StringField();
      password = PasswordField();
      bio = StringField();

When we serialize this a client app, we may want to strip out some data so it isn't leaked (such as their password).

    class PublicView(RestrictiveView):
        blacklist=['password']

This is an example view which blacklists access to the password field. You can instantiate these views with a Document that at least has a password field. Access to all other fields acts as normal, but when requesting a field under restriction, an exception is raised.

    >>> user = User(**{'username':'justinlilly',
    ...   'full_name':'Justin Lilly',
    ...   'password':'s3cur3!',
    ...   'bio': 'Born, now just trying to survive.'})
    >>> public_user = PublicView(user)
    >>> user.password
    "s3cur3!"
    >>> public_user.password
    Traceback (most recent call last):
      File "<stdin>", line 1, in <module>
    AttributeError: 'PublicView' object has no attribute 'password'

The other main type of views are transformative views. These views serve to alter the content of the underlying Document. A fun example of this might be a pirate speak view, but more serious versions might be a curse word filter, a view which will on-the-fly scale images into a scaled down size or compositing multiple fields into a synthetic field.

    class PirateSpeak(TransformativeView):
      transformable=['bio', 'full_name']

      def transform(self, field_name, value):
        if field_name == 'full_name':
          names = value.split()
          names[-1] = "Blackbeard"
          value = ' '.join(names)
        elif field_name == 'bio':
          value = pirateify(value)
        return value

How it works

RestrictiveView works by overriding the __getattr__() method. In the case of a whitelist, if the requested attribute isn't in the set of fields in the white list, an AttributeError is raised. If it is in the set, the call is proxied back to the instance of the Document.

TransformativeView is a bit different. When you attempt to get the backing attribute of the Document underlying the view, it is run through processing (assuming the attribute is in the transformable list), and the result is returned. The transform method is given a copy of the value from the underlying value.

Concerns

My biggest concern here is that you could build arbitrarily long chains of these views, which could cause a performance problem if each of them is just proxying back to the previous one to get an attribute. I haven't done much thinking as to how this could be mitigated, but doesn't seem untractable.

References

TODOs

  • TODO(justinlilly): Does requiring the backing model to have a password field matter? Should we just override getattr and if its password, blow up, otherwise let python handle it?

  • TODO(justinlilly): When you request a blacklisted field, should you get an exception, or a simple "doesn't exist, sorry" result?

  • TODO(justinlilly): How important is composeability with these Views?

  • TODO(justinlilly): Should TransformativeView do convention over configuration and make transform_bio(value) be for the bio call? I think I like that a bit better. Could also likely remove the transformable list.

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