django-rules

rules is a tiny but powerful app providing object-level permissions to Django, without requiring a database. At its core, it is a generic framework for building rule-based systems, similar to decision trees. It can also be used as a standalone library in other contexts and frameworks.

Features

rules has got you covered. rules is:

• Documented, tested, reliable and easy to use.
• Versatile. Decorate callables to build complex graphs of predicates. Predicates can be any type of callable -- simple functions, lambdas, methods, callable class objects, partial functions, decorated functions, anything really.
• A good Django citizen. Seamless integration with Django templates and the Django Admin for testing for object-level permissions.
• Efficient and smart. No need to mess around with a database to figure out whether John really wrote that book.
• Simple. Dive in the code. You'll need 10 minutes to figure out how it works.
• Powerful. rules comes complete with advanced features, such as invocation context and storage for arbitrary data, skipping evaluation of predicates under specific conditions, and more!

Requirements

rules requires Python 2.6/3.2 or newer. It can optionally integrate with Django, in which case requires Django 1.5 or newer.

How to install

Using pip:

$ pip install rules

Manually:

$ git clone https://github.com/dfunckt/django-rules.git
$ cd django-rules
$ python setup.py install

Using rules

rules is based on the idea that you maintain a dict-like object that maps string keys used as identifiers of some kind, to callables, called predicates. This dict-like object is actually an instance of RuleSet and the predicates are instances of Predicate.

Creating predicates

Let's ignore rule sets for a moment and go ahead and define a predicate. The easiest way is with the @predicate decorator:

>>> @rules.predicate
>>> def is_book_author(user, book):
...     return book.author == user
...
>>> is_book_author
<Predicate:is_book_author object at 0x10eeaa490>

This predicate will return True if the book's author is the given user, False otherwise.

Predicates can be created from any callable that accepts anything from zero to two positional arguments:

• fn(obj, target)
• fn(obj)
• fn()

This is their generic form. If seen from the perspective of authorization in Django, the equivalent signatures are:

• fn(user, obj)
• fn(user)
• fn()

Predicates can do pretty much anything with the given arguments, but must always return True if the condition they check is true, False otherwise. rules comes with several predefined predicates that you may read about later on in API Reference, that are mostly useful when dealing with authorization in Django.

Setting up rules

Let's pretend that we want to let authors edit or delete their books, but not books written by other authors. So, essentially, what determines whether an author can edit or can delete a given book is whether they are its author.

In rules, such requirements are modelled as rules. A rule is a map of a unique identifier (eg. "can edit") to a predicate. Rules are grouped together into a rule set. rules has two predefined rule sets:

• A default rule set storing shared rules.
• Another rule set storing rules that serve as permissions in a Django context.

So, let's define our first couple of rules, adding them to the shared rule set. We can use the is_book_author predicate we defined earlier:

>>> rules.add_rule('can_edit_book', is_book_author)
>>> rules.add_rule('can_delete_book', is_book_author)

Assuming we've got some data, we can now test our rules:

>>> from django.contrib.auth.models import User
>>> from books.models import Book
>>> guidetodjango = Book.objects.get(isbn='978-1-4302-1936-1')
>>> guidetodjango.author
<User: adrian>
>>> adrian = User.objects.get(username='adrian')
>>> rules.test_rule('can_edit_book', adrian, guidetodjango)
True
>>> rules.test_rule('can_delete_book', adrian, guidetodjango)
True

Nice... but not awesome.

Combining predicates

Predicates by themselves are not so useful -- not more useful than any other function would be. Predicates, however, can be combined using binary operators to create more complex ones. Predicates support the following operators:

• P1 & P2: Returns a new predicate that returns True if both predicates return True, otherwise False.
• P1 | P2: Returns a new predicate that returns True if any of the predicates returns True, otherwise False.
• P1 ^ P2: Returns a new predicate that returns True if one of the predicates returns True and the other returns False, otherwise False.
• ~P: Returns a new predicate that returns the negated result of the original predicate.

Suppose the requirement for allowing a user to edit a given book was for them to be either the book's author, or a member of the "editors" group. Allowing users to delete a book should still be determined by whether the user is the book's author.

With rules that's easy to implement. We'd have to define another predicate, that would return True if the given user is a member of the "editors" group, False otherwise. The built-in is_group_member factory will come in handy:

>>> is_editor = rules.is_group_member('editors')
>>> is_editor
<Predicate:is_group_member:editors object at 0x10eee1350>

We could combine it with the is_book_author predicate to create a new one that checks for either condition:

>>> is_book_author_or_editor = is_book_author | is_editor
>>> is_book_author_or_editor
<Predicate:(is_book_author | is_group_member:editors) object at 0x10eee1390>

We can now update our can_edit_book rule:

>>> rules.add_rule('can_edit_book', is_book_author_or_editor)
Traceback (most recent call last):
    ...
KeyError: A rule with name `can_edit_book` already exists
>>> rules.remove_rule('can_edit_book')
>>> rules.add_rule('can_edit_book', is_book_author_or_editor)
>>> rules.test_rule('can_edit_book', adrian, guidetodjango)
True
>>> rules.test_rule('can_delete_book', adrian, guidetodjango)
True

Let's see what happens with another user:

>>> martin = User.objects.get(username='martin')
>>> list(martin.groups.values_list('name', flat=True))
['editors']
>>> rules.test_rule('can_edit_book', martin, guidetodjango)
True
>>> rules.test_rule('can_delete_book', martin, guidetodjango)
False

Awesome.

So far, we've only used the underlying, generic framework for defining and testing rules. This layer is not at all specific to Django; it may be used in any context. There's actually no import of anything Django-related in the whole app (except in the rules.templatetags module). rules however can integrate tightly with Django to provide authorization.

Using rules with Django

rules is able to provide object-level permissions in Django. It comes with an authorization backend and a couple template tags for use in your templates.

Permissions

In rules, permissions are a specialised type of rules. You still define rules by creating and combining predicates. These rules however, must be added to a permissions-specific rule set that comes with rules so that they can be picked up by the rules authorization backend.

Creating permissions

The convention for naming permissions in Django is app_label.action_object, and we like to adhere to that. Let's add rules for the books.change_book and books.delete_book permissions:

>>> rules.add_perm('books.change_book', is_book_author | is_editor)
>>> rules.add_perm('books.delete_book', is_book_author)

See the difference in the API? add_perm adds to a permissions-specific rule set, whereas add_rule adds to a default shared rule set. It's important to know however, that these two rule sets are separate, meaning that adding a rule in one does not make it available to the other.

Checking for permission

Let's go ahead and check whether adrian has change permission to the guidetodjango book:

>>> adrian.has_perm('books.change_book', guidetodjango)
False

When you call the User.has_perm method, Django asks each backend in settings.AUTHENTICATION_BACKENDS whether a user has the given permission for the object. When queried for object permissions, Django's default authentication backend always returns False. rules comes with an authorization backend, that is able to provide object-level permissions by looking into the permissions-specific rule set.

Let's add the rules authorization backend in settings:

AUTHENTICATION_BACKENDS = (
    'rules.permissions.ObjectPermissionBackend',
    'django.contrib.auth.backends.ModelBackend',
)

Now, checking again gives adrian the required permissions:

>>> adrian.has_perm('books.change_book', guidetodjango)
True
>>> adrian.has_perm('books.delete_book', guidetodjango)
True
>>> martin.has_perm('books.change_book', guidetodjango)
True
>>> martin.has_perm('books.delete_book', guidetodjango)
False

Rules and permissions in templates

rules comes with two template tags to allow you to test for rules and permissions in templates.

Add rules to your INSTALLED_APPS:

INSTALLED_APPS = (
    # ...
    'rules',
)

Then, in your template:

{% load rules %}

{% has_perm 'books.change_book' author book as can_edit_book %}
{% if can_edit_book %}
    ...
{% endif %}

{% test_rule 'has_super_feature' user as has_super_feature %}
{% if has_super_feature %}
    ...
{% endif %}

Rules and permissions in the Admin

If you've setup rules to be used with permissions in Django, you're almost set to also use rules to authorize any add/change/delete actions in the Admin. The Admin asks for four different permissions, depending on action:

• <app_label>.add_<modelname>
• <app_label>.change_<modelname>
• <app_label>.delete_<modelname>
• <app_label>

The first three are obvious. The fourth is the required permission for an app to be displayed in the Admin's "dashboard". Here's some rules for our imaginary books app as an example:

>>> rules.add_perm('books', rules.always_allow)
>>> rules.add_perm('books.add_book', is_staff)
>>> rules.add_perm('books.change_book', is_staff)
>>> rules.add_perm('books.delete_book', is_staff)

Object permissions

Django Admin does not support object-permissions, in the sense that it will never ask for permission to perform an action on an object, only whether a user is allowed to act on (any) instances of a model.

If you'd like to tell Django whether a user has permissions on a specific object, you'd have to override the following methods of a model's ModelAdmin:

• has_change_permission(user, obj=None)
• has_delete_permission(user, obj=None)

Note: There's also has_add_permission(user) but is not relevant here.

rules comes with a custom ModelAdmin subclass, rules.contrib.admin.ObjectPermissionsModelAdmin, that overrides these methods to pass on the edited model instance to the authorization backends, thus enabling permissions per object in the Admin:

# books/admin.py
from django.contrib import admin
from rules.contrib.admin import ObjectPermissionsModelAdmin
from .models import Book

class BookAdmin(ObjectPermissionsModelAdmin):
    pass

admin.site.register(Book, BookAdmin)

Now this allows you to specify permissions like this:

>>> rules.add_perm('books', rules.always_allow)
>>> rules.add_perm('books.add_book', has_author_profile)
>>> rules.add_perm('books.change_book', is_book_author_or_editor)
>>> rules.add_perm('books.delete_book', is_book_author)

Advanced features

Custom rule sets

You may create as many rule sets as you need:

>>> features = rules.RuleSet()

And manipulate them by adding, removing, querying and testing rules:

>>> features.rule_exists('has_super_feature')
False
>>> is_special_user = rules.is_group_member('special')
>>> features.add_rule('has_super_feature', is_special_user)
>>> 'has_super_feature' in features
True
>>> features['has_super_feature']
<Predicate:is_group_member:special object at 0x10eeaa500>
>>> features.test_rule('has_super_feature', adrian)
True
>>> features.remove_rule('has_super_feature')

Note however that custom rule sets are not available in Django templates -- you need to provide integration yourself.

Invocation context

A new context is created as a result of invoking Predicate.test() and is only valid for the duration of the invocation. A context is a simple dict that you can use to store arbitrary data, (eg. caching computed values, setting flags, etc.), that can be used by predicates later on in the chain. Inside a predicate function it can be used like so:

>>> @predicate
... def mypred(a, b):
...     value = compute_expensive_value(a)
...     mypred.context['value'] = value
...     return True

Other predicates can later use stored values:

>>> @predicate
... def myotherpred(a, b):
...     value = myotherpred.context.get('value')
...     if value is not None:
...         return do_something_with_value(value)
...     else:
...         return do_something_without_value()

Predicate.context provides a single args attribute that contains the arguments as given to test() at the beginning of the invocation.

Binding self

In a predicate's function body, you can refer to the predicate instance itself by its name, eg. is_book_author. Passing bind=True as a keyword argument to the predicate decorator will let you refer to the predicate with self, which is more convenient. Binding self is just syntactic sugar. As a matter of fact, the following two are equivalent:

>>> @predicate
... def is_book_author(user, book):
...     if is_book_author.context.args:
...         return user == book.author
...     return False

>>> @predicate(bind=True)
... def is_book_author(self, user, book):
...     if self.context.args:
...         return user == book.author
...     return False

Skipping predicates

You may skip evaluation by calling the skip() method of your predicate:

>>> @predicate(bind=True)
... def is_book_author(self, user, book):
...     if self.context.args:
...         return user == book.author
...     else:
...         self.skip()

skip() raises a special exception that is caught by rules to signal that a predicate need not be evaluated, thus leaving the predicate result up to that point unchanged.

Best practices

Before you can test for rules, these rules must be registered with a rule set, and for this to happen the modules containing your rule definitions must be imported.

For complex projects with several predicates and rules, it may not be practical to define all your predicates and rules inside one module. It might be best to split them among any sub-components of your project. In a Django context, these sub-components could be the apps for your project.

On the other hand, because importing predicates from all over the place in order to define rules can lead to circular imports and broken hearts, it's best to further split predicates and rules in different modules.

If using Django 1.7 and later, rules may optionally be configured to autodiscover rules.py modules in your apps and import them at startup. To have rules do so, just edit your INSTALLED_APPS setting:

INSTALLED_APPS = (
    # replace 'rules' with:
    'rules.apps.AutodiscoverRulesConfig',
)

Note: On Python 2, you must also add the following to the top of your rules.py file, or you'll get import errors trying to import django-rules itself:

from __future__ import absolute_import

API Reference

Everything is accessible from the root rules module.

Class rules.Predicate

You create Predicate instances by passing in a callable:

>>> def is_book_author(user, book):
...     return book.author == user
...
>>> pred = Predicate(is_book_author)
>>> pred
<Predicate:is_book_author object at 0x10eeaa490>

You may optionally provide a different name for the predicate that is used when inspecting it:

>>> pred = Predicate(is_book_author, name='another_name')
>>> pred
<Predicate:another_name object at 0x10eeaa490>

Also, you may optionally provide bind=True in order to be able to access the predicate instance with self:

>>> def is_book_author(self, user, book):
...     if self.context.args:
...         return user == book.author
...     return False
...
>>> pred = Predicate(is_book_author, bind=True)
>>> pred
<Predicate:is_book_author object at 0x10eeaa490>

Instance methods

test(obj=None, target=None)

Returns the result of calling the passed in callable with zero, one or two positional arguments, depending on how many it accepts.

Class rules.RuleSet

RuleSet extends Python's built-in dict type. Therefore, you may create and use a rule set any way you'd use a dict.

Instance methods

add_rule(name, predicate)

Adds a predicate to the rule set, assigning it to the given rule name. Raises KeyError if another rule with that name already exists.

remove_rule(name)

Remove the rule with the given name. Raises KeyError if a rule with that name does not exist.

rule_exists(name)

Returns True if a rule with the given name exists, False otherwise.

test_rule(name, obj=None, target=None)

Returns the result of calling predicate.test(obj, target) where predicate is the predicate for the rule with the given name. Returns False if a rule with the given name does not exist.

Decorators

@predicate

Decorator that creates a predicate out of any callable:

>>> @predicate
... def is_book_author(user, book):
...     return book.author == user
...
>>> is_book_author
<Predicate:is_book_author object at 0x10eeaa490>

Customising the predicate name:

>>> @predicate(name='another_name')
... def is_book_author(user, book):
...     return book.author == user
...
>>> is_book_author
<Predicate:another_name object at 0x10eeaa490>

Binding self:

>>> @predicate(bind=True)
... def is_book_author(self, user, book):
...     if 'user_has_special_flag' in self.context:
...         return self.context['user_has_special_flag']
...     return book.author == user

Predefined predicates

always_allow(), always_true()

Always returns True.

always_deny(), always_false()

Always returns False.

is_authenticated(user)

Returns the result of calling user.is_authenticated(). Returns False if the given user does not have an is_authenticated method.

is_superuser(user)

Returns the result of calling user.is_superuser. Returns False if the given user does not have an is_superuser property.

is_staff(user)

Returns the result of calling user.is_staff. Returns False if the given user does not have an is_staff property.

is_active(user)

Returns the result of calling user.is_active. Returns False if the given user does not have an is_active property.

is_group_member(*groups)

Factory that creates a new predicate that returns True if the given user is a member of all the given groups, False otherwise.

Shortcuts

Managing the shared rule set

add_rule(name, predicate)

Adds a rule to the shared rule set. See RuleSet.add_rule.

remove_rule(name)

Remove a rule from the shared rule set. See RuleSet.remove_rule.

rule_exists(name)

Returns whether a rule exists in the shared rule set. See RuleSet.rule_exists.

test_rule(name, obj=None, target=None)

Tests the rule with the given name. See RuleSet.test_rule.

Managing the permissions rule set

add_perm(name, predicate)

Adds a rule to the permissions rule set. See RuleSet.add_rule.

remove_perm(name)

Remove a rule from the permissions rule set. See RuleSet.remove_rule.

perm_exists(name)

Returns whether a rule exists in the permissions rule set. See RuleSet.rule_exists.

has_perm(name, user=None, obj=None)

Tests the rule with the given name. See RuleSet.test_rule.

Licence

django-rules is distributed under the MIT licence.

Copyright (c) 2014 Akis Kesoglou

Permission is hereby granted, free of charge, to any person obtaining a copy of this software and associated documentation files (the "Software"), to deal in the Software without restriction, including without limitation the rights to use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies of the Software, and to permit persons to whom the Software is furnished to do so, subject to the following conditions:

The above copyright notice and this permission notice shall be included in all copies or substantial portions of the Software.

THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.