This is a pre-production deployment of Warehouse. Changes made here affect the production instance of PyPI (pypi.python.org).
Help us improve Python packaging - Donate today!

A predicate class constructed like Django Q objects, used to test whether a new or modified model would match a query

Project Description

django-predicate provides a Q like object to facilitate the question: “would this model instance be part of a query” but without running the query or even saving the object.

Quickstart

Install django-predicate:

pip install django-predicate

Then use the P object just as you would Q objects:

from predicate import P

p = P(some_field__startswith="hello", age__gt=20)

You can then call the eval method with a model instance to check whether it passes the conditions:

model_instance = MyModel(some_field="hello there", age=21)
other_model_instance = MyModel(some_field="hello there", age=10)
p.eval(model_instance)
>>> True
p.eval(other_model_instance)
>>> False

or you can use Python’s in operator.

model_instance in p
>>> True

Even though a predicate is not a true container class - it can be used as (and was designed as being) a virtual “set” of objects that meets some condiiton.

Like Q objects, P objects can be &’ed and |’ed together to form more complex logic groupings.

In fact, P objects are actually a subclass of Q objects, so you can use them in queryset filter statements:

qs = MyModel.objects.filter(p)

P objects also support QuerySet-like filtering operations that can be applied to an arbitrary iterable: P.get(iterable), P.filter(iterable), and P.exclude(iterable):

model_instance = MyModel(some_field="hello there", age=21)
other_model_instance = MyModel(some_field="hello there", age=10)
p.filter([model_instance, other_model_instance]) == [model_instance]
>>> True
p.get([model_instance, other_model_instance]) == model_instance
>>> True
p.exclude([model_instance, other_model_instance]) == [other_model_instance]
>>> True

If you have a situation where you want to use querysets and predicates based on the same conditions, it is far better to start with the predicate. Because of the way querysets assume a SQL context, it is non-trivial to reverse engineer them back into a predicate. However as seen above, it is very straightforward to create a queryset based on a predicate.

Release History

Release History

This version
History Node

1.4.0

History Node

1.3.1

History Node

1.3.0

History Node

1.2.2

History Node

1.2.1

History Node

1.2.0

History Node

1.1.0

History Node

1.0.1

History Node

1.0.0

History Node

0.1.1

History Node

0.1

Download Files

Download Files

Download the file for your platform. If you're not sure which to choose, learn more about installing packages.

File Name & Checksum SHA256 Checksum Help Version File Type Upload Date
django-predicate-1.4.0.tar.gz (45.0 kB) Copy SHA256 Checksum SHA256 Source Apr 9, 2016

Supported By

WebFaction WebFaction Technical Writing Elastic Elastic Search Pingdom Pingdom Monitoring Dyn Dyn DNS Sentry Sentry Error Logging CloudAMQP CloudAMQP RabbitMQ Heroku Heroku PaaS Kabu Creative Kabu Creative UX & Design Fastly Fastly CDN DigiCert DigiCert EV Certificate Rackspace Rackspace Cloud Servers DreamHost DreamHost Log Hosting