django-konst 3.0.0
Convenient constants fields for Django
License: MIT

# Django Konst

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## django-konst

`django-konst` is a utility for Django that makes the definition, use and storage of both integer and string based constants easy and readable. It avoids passing of constants
into template contexts and makes evaluation concise and readable.

It also makes exposure of these constants via forms and DRF serializers simple.

### Definition ###

Constants can be defined with friendly names, backed by either integers or text.

#### Constants ####

```python
from konst import Constant, Constants

# states backed by integers
states = Constants(
    Constant(pending=0),
    Constant(active=1),
    Constant(inactive=2)
)

# and another set of constants backed by strings
colours = Constants(
    Constant(red="FF0000"),
    Constant(green="00FF00"),
    Constant(yellow="FFFF80"),
    Constant(white="FFFFFF")
)

```

#### Constant groups ####

At times, it will be necessary to group constants and test membership within that group.
To achieve this, `django-konst` provides a `ConstantGroup` class.

```python
from konst import Constant, ConstantGroup

# states backed by integers
states = Constants(
    Constant(active=0),
    Constant(cancelled_ontime=1),
    Constant(cancelled_late=2),
    ConstantGroup(
        "cancelled",
        ("cancelled_ontime", "cancelled_late")
    )
)

```

#### Within a model ####

While not strictly necessary, it is advisable to effectively namespace your constants
by defining them in the scope of a model definition. This means you have your constants
wherever you have the model class, as well as any model instance.


```python
from django.db import models
from django.utils.translation import gettext_lazy as _

from konst import Constant, ConstantGroup, Constants
from konst.models.fields import (
    ConstantChoiceCharField,
    ConstantChoiceField
)

class Apple(models.Model):

    purposes = Constants(
        Constant(cooking=0, label=_("Cook me!")),
        Constant(eating=1, label=_("Eat me!")),
        Constant(juicing=2, label=_("Juice me!")),
        Constant(ornamental=3, label=_("Just look how pretty I am!")),
        ConstantGroup(
            "culinary", ("cooking", "eating", "juicing")
        )
    )
    colours = Constants(
        Constant(red="FF0000", label=_("red")),
        Constant(green="00FF00", label=_("green")),
        Constant(yellow="FFFF80", label=_("yellow")),
        Constant(white="FFFFFF", label=_("white")),
    )

    name = models.CharField(max_length=30)
    purpose = ConstantChoiceField(constants=purposes)
    colour = ConstantChoiceCharField(constants=colours, max_length=30)

```


### Use ###

The entire point of this library is to make the use of constants defined in this way
easy and concise.

#### In code ####

```python
apple = Apple.objects.get(name='Granny Smith')
apple.purpose.cooking
True
apple.colour.red
False
apple.colour.green
True

# we don't care about the specific purpose, just whether it is as food
# or not, so use the ConstantGroup!
apple.purpose.culinary
True

```

#### In templates ####

```
{% if apple.purpose.eating %}
    You should bite this {{ apple.name }}!
{% endif %}
```

#### With Django's ORM ####

```python
red_apples = Apple.objects.filter(colour=Apple.colours.red)
culinary_apples = Apple.objects.filter(
    purpose__in=Apple.purposes.culinary
)

```

#### Django Form ####

```python
from django import forms

from konst.forms.fields import ConstantChoiceField

class SomeForm(forms.Form):
    purpose = ConstantChoiceField(constants=Apple.purposes) 
```

#### With Django Rest Framework ####

Using the `konst.extras.drf.fields.ConstantChoiceField` serializer field with the
Django Rest Framework it is possible to both output and receive constant values.

```python
from konst.extras.drf.fields import ConstantChoiceField

from rest_framework import serializers


class AppleSerializer(serializers.ModelSerializer):

    purpose = ConstantChoiceField(Apple.purposes)
    colour = ConstantChoiceField(Apple.colours)

    class Meta:
        model = Apple
        fields = (
            "name", "purpose", "colour"
        )


# let's see how it handles bad values
serializer = AppleSerializer(
    data={
        "name": "Fuji",
        "colour": "blue",
        "purpose": "dicing"
    }
)
serializer.is_valid()
False
serializer.errors
{
    'colour': [u'"blue" is not a valid choice.'],
    'purpose': [u'"dicing" is not a valid choice.']
}


# and now how it handles some good values
serializer = AppleSerializer(
    data={
        "name": "Fuji",
        "colour": "red",
        "purpose": "eating"
    }
)
serializer.is_valid()
True


# let's create a database entry!
instance = serializer.save()


# and now our instance can be interacted with neatly
instance.colour.red
True


# finally, let's see how this looks when rendering JSON
AppleSerializer(instance=instance).data
{
    "name": "Fuji",
    "colour": "red",
    "purpose": "eating"
}

```

## Gotchas

### Setting a field on a model to the value of a constant rather than the Constant object

When the Django ORM instantiates a model instance from the database, it will ensure that any `ConstantChoiceField` or `ConstantChoiceCharField` fields have values set to `Constant` instances.

If you create or modify a model instance using a raw, hard-coded constant value, you're likely to hit errors along the lines of "AttributeError: 'int' object has no attribute 'attribute-name'".

Take the example below where the caller sets the colour of a model instance to the underlying value for the `Constant`:

```python
apple = Apple.objects.get(name='Granny Smith')
apple.purpose = 1  # constant for `eating` .. I know, who on earth would eat a granny smith straight up?!
apple.save()
```

Django will happily persist the data correctly, but let's say you have a `post_save` signal (or any other code, really) that does something further with the instance:

```python
from django.db.models.signals import post_save
from django.dispatch import receiver

from .models import Apple

@receiver(post_save, sender=Apple)
def apple_updated_receiver(sender, instance, created, *args, **kwargs):
    if not created and instance.purpose.eating:
        print(f"Oh my, {instance.name} is now for eating!")

```

This code will sadly raise `AttributeError: 'int' object has no attribute 'purpose'`, as `instance.purpose` is just the integer we set it to before saving.

The good news is that you can avoid this by *always* setting `django-konst` fields on instances to `Constant` instances such that downstream code can happily handle the instance as if it'd come straight out of the database:

```python
apple = Apple.objects.get(name='Granny Smith')
apple.purpose = Apple.purposes.eating
apple.save()
```

Regardless of whether you're using `django-konst` for your constants, it's good practice to not hard-code constant values in order to avoid subtle mistakes and to ease changes in the future.


## Contribute

`django-konst` supports a variety of Python and Django versions. It's best if you test each one of these before committing. Our [Circle CI Integration](https://circleci.com) will test these when you push but knowing before you commit prevents from having to do a lot of extra commits to get the build to pass.

### Environment Setup

In order to easily test on all these Pythons and run the exact same thing that CI will execute you'll want to setup [pyenv](https://github.com/yyuu/pyenv) and install the Python versions outlined in [tox.ini](tox.ini).

If you are on Mac OS X, it's recommended you use [brew](http://brew.sh/). After installing `brew` run:

```bash
brew install pyenv pyenv-virtualenv pyenv-virtualenvwrapper
```

Next, install the various python versions we want to test against and create a virtualenv specifically for `django-konst`:

```bash
pyenv install 3.6.10
pyenv install 3.7.6
pyenv install 3.8.1
pyenv virtualenv 3.8.1 konst
pyenv activate konst
pip install detox
pyenv shell konst 3.6.10 3.7.6
```

Now ensure the `konst` virtualenv is activated, make the other python versions also on our path, and run the tests!


```bash
pyenv shell konst 3.6.10 3.7.6
detox
```

This will execute the test environments in parallel as defined in the `tox.ini`.