django-pint-field 0.3.5

Pint Fields for Django and Postgres

django-pint-field

Use pint with Django's ORM.

If you want to store quantities (1 gram, 3 miles, 8.120391 angstroms, etc) in a model, edit them in forms, and have the ability to convert to other quantities in your django projects, this is the package for you!

This package is modified from the fantastic django-pint with different goals. Unlike django-pint, in this project we use a composite Postgres field to store both the magnitude and the user's desired units, along with the equivalent value in base units. This third piece of date - the base units - makes it possible to conduct lookups comparing one instance that might be specified in "grams" with another that may be specified in "pounds", but display each instance in the units that the user desires. The units your users want to use are the units they see, while still allowing accurate comparisons of one quantity to another.

For this reason, the project only works with Postgresql databases.

Install

pip install django_pint_field

Add "django_pint_field", to your list of installed apps.

Run python manage.py migrate django_pint_field

Usage

Assuming we are starting with the following model:

from decimal import Decimal
from django.db import models
from django.db.models import DecimalField

from django_pint_field.models import IntegerPintField


class IntegerPintFieldSaveModel(models.Model):
    name = models.CharField(max_length=20)
    weight = IntegerPintField("gram")

    def __str__(self):
        return str(self.name)

We can do the following:

from decimal import Decimal
from django_pint_field.units import ureg

from .models import IntegerPintFieldSaveModel

Quantity = ureg.Quantity

# Start by creating a few Pint Quantity objects
extra_small = Quantity(1 * ureg.gram)
small = Quantity(10 * ureg.gram)
medium = Quantity(100 * ureg.gram)
large = Quantity(1000 * ureg.gram)
extra_large = Quantity(10000 * ureg.gram)

# Create a couple objects
IntegerPintFieldSaveModel.objects.create(name="small", weight=small)
IntegerPintFieldSaveModel.objects.create(name="large", weight=large)

In [1]: IntegerPintFieldSaveModel.objects.first()
Out[1]: <IntegerPintFieldSaveModel: small>

In [2]: IntegerPintFieldSaveModel.objects.first().weight
Out[2]: 10 <Unit('gram')>

In [3]: IntegerPintFieldSaveModel.objects.first().weight.magnitude
Out[3]: 10

In [4]: IntegerPintFieldSaveModel.objects.first().weight.units
Out[4]: <Unit('gram')>

Valid Lookups

Other lookups will be added in the future. Currently available are:

• exact
• gt
• gte
• lt
• lte
• range
• isnull

# Perform some queries
IntegerPintFieldSaveModel.objects.filter(weight__gt=medium)
<QuerySet [<IntegerPintFieldSaveModel: large>]>

IntegerPintFieldSaveModel.objects.filter(weight__gt=extra_small)
<QuerySet [<IntegerPintFieldSaveModel: small>, <IntegerPintFieldSaveModel: large>]>

IntegerPintFieldSaveModel.objects.filter(weight__gte=small)
<QuerySet [<IntegerPintFieldSaveModel: small>, <IntegerPintFieldSaveModel: large>]>

IntegerPintFieldSaveModel.objects.filter(weight__range=(small, medium))
<QuerySet [<IntegerPintFieldSaveModel: small>]>

Aggregates

A number of aggregates have been implemented for the Django Pint Fields. Functionally they perform for Pint Fields the same way django's default aggregates work for other field types, and each is prepended with "Pint". The aggregates include:

• PintAvg
• PintCount
• PintMax
• PintMin
• PintStdDev
• PintSum
• PintVariance

from django_pint_field.aggregates import PintAvg, PintCount, PintMax, PintMin, PintStdDev, PintSum, PintVariance

# Perform some queries
IntegerPintFieldSaveModel.objects.aggregate(PintAvg('weight'))
{'weight__pintavg': Decimal('0.50500000000000000000') <Unit('kilogram')>}

IntegerPintFieldSaveModel.objects.aggregate(PintCount('weight'))
{'weight__pintcount': 2}

IntegerPintFieldSaveModel.objects.aggregate(PintMax('weight'))
{'weight__pintmax': Decimal('1.0') <Unit('kilogram')>}

IntegerPintFieldSaveModel.objects.aggregate(PintMin('weight'))
{'weight__pintmin': Decimal('0.01') <Unit('kilogram')>}

IntegerPintFieldSaveModel.objects.aggregate(PintStdDev('weight'))
{'weight__pintstddev': Decimal('0.49500000000000000000') <Unit('kilogram')>}

IntegerPintFieldSaveModel.objects.aggregate(PintSum('weight'))
{'weight__pintsum': Decimal('1.01') <Unit('kilogram')>}

IntegerPintFieldSaveModel.objects.aggregate(PintVariance('weight'))
{'weight__pintvariance': Decimal('0.24502500000000000000') <Unit('kilogram')>}

Use with Django Rest Framework

from django_pint_field.rest import IntegerPintRestField

class DjangoPintFieldSerializer(serializers.ModelSerializer):
    # Let DRF know which type of serializer field to use
    weight = IntegerPintRestField()

    class Meta:
        model = IntegerPintFieldSaveModel
        fields = ["name", "weight"]

Creating your own units

You can create your own pint units if the default units in pint are not sufficient.

Anywhere within your project (ideally in settings or a file adjacent to settings), define the custom unit registry by importing Pint's default UnitRegistry and extending it:

from pint import UnitRegistry

custom_ureg = UnitRegistry()
custom_ureg.define("custom = [custom]")
custom_ureg.define("kilocustom = 1000 * custom")

Then add the custom registry to your app's settings.py:

DJANGO_PINT_FIELD_UNIT_REGISTER = custom_ureg

Model Fields

• IntegerPintField: Stores a pint measurement as an integer (-2147483648 to 2147483647).
• BigIntegerPintField: Stores a pint measurement as a big integer (-9223372036854775808 to 9223372036854775807).
• DecimalPintField: Stores a pint measurement as a decimal. Like Django's DecimalField, DecimalPintField takes required max_digits and decimal_places parameters.

Form Fields

• IntegerPintFormField: Used in forms with IntegerPintField and BigIntegerPintField.
• DecimalPintFormField: Used in forms with DecimalPintField.

Widgets

• PintFieldWidget: Default widget for all django pint field types.
• TabledPintFieldWidget: Provides a table showing conversion to each of the unit_choices.

Example usage:

class WeightModel(models.Model):
    decimal_weight = DecimalPintField(
        "gram",
        blank=True,
        null=True,
        max_digits=10,
        decimal_places=2,
        unit_choices=[
            "kilogram",
            "milligram",
            "pounds"
        ],
    )

class TabledWeightForm(forms.ModelForm):
    class Meta:
        model = WeightModel
        fields = "__all__"

    def __init__(self, *args, **kwargs):
        super().__init__(*args, **kwargs)

        # If we want to use the tabled widget, we need to pass the 
        #   default_unit and unit_choices to the new widget
        default_unit = self.fields["decimal_weight"].default_unit
        unit_choices = self.fields["decimal_weight"].unit_choices
        self.fields["decimal_weight"].widget = TabledPintFieldWidget(
            default_unit=default_unit, unit_choices=unit_choices
        )

The template for this widget is located at https://github.com/jacklinke/django-pint-field/blob/main/django_pint_field/templates/tabled_django_pint_field_widget.html

If you want to override the template, add your own template in your project at: "templates/django_pint_field/tabled_django_pint_field_widget.html"

DRF Serializer Fields

• IntegerPintRestField: Used in DRF with IntegerPintField and BigIntegerPintField.
• DecimalPintRestField: Used in DRF with DecimalPintField.

Settings

DJANGO_PINT_FIELD_DECIMAL_PRECISION

Determines whether django_pint_field should automatically set the python decimal precision for the project. If an integer greater than 0 is provided, the decimal context precision for the entire project will be set to that value. Otherwise, the precision remains at the default (usually 28).
* Type: int * Default: 0

DJANGO_PINT_FIELD_UNIT_REGISTER

The Unit Registry to use in the project. Defaults to pint.UnitRegistry.
* Type: int * Default: 0

Rounding modes (upcoming feature)

decimal.ROUND_CEILING Round towards Infinity.

decimal.ROUND_DOWN Round towards zero.

decimal.ROUND_FLOOR Round towards -Infinity.

decimal.ROUND_HALF_DOWN Round to nearest with ties going towards zero.

decimal.ROUND_HALF_EVEN Round to nearest with ties going to nearest even integer.

decimal.ROUND_HALF_UP Round to nearest with ties going away from zero.

decimal.ROUND_UP Round away from zero.

decimal.ROUND_05UP Round away from zero if last digit after rounding towards zero would have been 0 or 5; otherwise round towards zero.

Read more about rounding modes for decimals at the decimal docs

Use the test app with docker compose

Set up a virtual environment (recommended)

Make a virtual environment named .venv:

python3 -m venv .venv

Activate the new environment:

source .venv/bin/activate

Install dependencies

Requires poetry. If you do not yet have it installed, run pip install poetry.

poetry install --with dev

Build and bring up

docker compose build
docker compose run django python manage.py migrate
docker compose run django python manage.py createsuperuser
docker compose up -d

Navigate to 127.0.0.1:8000

Test (assuming you have already performed build)

docker compose run django python manage.py test

Run psql on the Postgres database

docker compose exec postgres psql -U postgres

ToDos:

• Implement rounding modes
• Think through what it would take to build range types for these fields
• Add extensible widget template