Introduction

django-expression-fields lets your users type a mathematical expression in a form field. Python does the math and stores the result in the database. For example, suppose you have a model to track Things, like this:

class Thing(models.Model):
	cost = models.DecimalField(
		max_digits=5, decimal_places=2, null=True, blank=True)

Suppose Things come in packs of 12 for $7.99. Your users have to do some math to fill in the cost of a single Thing, $0.67.

But not with an expression field! Create your form like this:

class ThingForm(forms.Form):
	cost = DecimalExpressionField(
		max_digits=5, decimal_places=2, required=False)

Now your user can simply type 7.99/12 in the field and Python will do the math for them!

Requirements and Installation

Right now, the project has no dependencies outside of Django itself. It has been updated to work with Python 3.9-3.10 and Django 3.2-4.0.

• pip install django-expression-fields
• Add expression_fields to your INSTALLED_APPS.

Use

from django import forms
from expression_fields.fields import DivideDecimalField

class MyForm(forms.Form):
	cost = DecimalExpressionField(
		max_digits=5, decimal_places=2, required=False)

Tests

./run-tests.sh.

Limitations

• Only DecimalExpressionField exists today. I later intend to build Integer and Float expression fields as well.
• This field slightly changes the behavior of the existing DecimalField: Inputs that would rejected for having too many digits after the decimal are instead rounded.
• For historical reasons, there's a DivideDecimalField which allows a single division sign.

Contributions

I built this little project to satisfy a personal need, but thought it might be useful enough for others. If you have contributions, please don't hesitate to send a PR. Let's keep the tests passing and all will be well.