django-predicate 2.0.1

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

Notice

2022-04-27: This library is currently unmaintained, since I no longer use Django. If there is any interest, I’m still happy to review pull requests and consider adding maintainers. - @lucaswiman

django-predicate

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.

Development

These instructions assume you have Postgres installed and all referenced Python versions installed and activated, e.g. with Pyenv.

To run the tests locally, do the following:

1. Clone this repo.

2. Start Postgres in the background for the Postgres tests with postgres -D ~/postgres.

3. Activate all tested Python versions used in this repo: pyenv local 3.6 3.7 3.10.

4. (Optional) Create a virtualenv and activate it: python -m venv .venv, source .venv/bin/activate.

5. (Optional) Upgrade pip to prevent platform issues pip install --upgrade pip

6. Install Tox for running tests: pip install tox.

7. Run all tests by issuing command tox with no arguments.

Changelog

2.0.1

• Added support for Python 3.7 and 3.10, and Django 2.2, 3.2, and 4.1 (see tox.ini for compatible Python/Django version tuples).

• Converted test runner from Nose to Pytest.

• BREAKING Dropped support for Python 2.7, Python 3.5, and Django 1.9.

2.0.0 (Unreleased)

This version was pushed to Master but was not pushed to PyPI.

• Added deprecation warning to README.

• Added Travis CI config.

• BREAKING Dropped support for Django 1.7 and 1.8.

1.4.0

This version and below aren’t covered in this changelog.