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Use complex constants built with 'candv' library instead of standard 'choices' fields for 'Django' models.

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Use complex constants built with candv library instead of standard choices fields for Django models.

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Install from PyPI:

$ pip install django-candv-choices

Problem overview

Well, you need to define some constant choices for your Django model field. Let’s start from defining constants themselves:

from django.utils.translation import ugettext_lazy as _


    (AUTH_TYPE_BASIC, _("HTTP Basic Authentication")),
    (AUTH_TYPE_DIGEST, _("HTTP Digest Authentication ")),
    (AUTH_TYPE_CLIENT_CERT, _("HTTPS Client Authentication ")),
    (AUTH_TYPE_FORM, _("Form Based Authentication ")),

Here we define constant names and attach verbose names to them. Bloated definition, no docstring for constants group, no docstings per constant. What if you need to define some help text per constant? 4 more definitions? Well, then just imagine, how you will attach them. And what about other attributes? And what about adding some methods for constants? How about getting constant by its name? By value? And how about performing some operations on the whole constants group?

Only at this point you may end up with one big module which will work only with one group of constants. And this work will be a big pain.

But OK, let’s go further and define some DB model field:

from django.db import models
from django.utils.translation import ugettext_lazy as _

from . constants import AUTH_TYPES, AUTH_TYPE_BASIC

class Request(models.Model):

    auth_type = models.CharField(
        verbose_name=_("Auth type"),
        help_text=_("Example of default constants"),

3 things to mention here:

  • you have to import constant group itself;

  • you may have to import dafault value too;

  • you need go back to constants definition, iterate over each constant, calculate its length and select the longest value to pass it as max_length argument. And don’t try to make a mistake, or you will be punished otherwise.

I use CharField here intentionally. It can be good to use IntegerField, PositiveSmallIntegerField and so on, but it is very probable that you will be willing someone to kill you due to hidden bugs.

Now it’s showtime! Let’s render our field:

{% for r in requests %}
  <li>{{ r.auth_type }}</li>
{% endfor %}

What do you see? BASIC, DIGEST, FORM, etc. Oops! How to get our human messages like HTTP Basic Authentication?

You need to convert constants group to dict and pass it to template’s context! But wait, this is not the end. You can not access dict values directly within templates. You need to create a library of template tags, register a filter and load the library to template:

# templatetags/
from django import template

register = template.Library()

def lookup(d, key):
    return d[key]
{% load custom_tags %}
{% for r in requests %}
  <li>{{ AUTH_TYPES|lookup:r.auth_type }}</li>
{% endfor %}

This is madness!


The solution is to use candv and this library. The former allows you to define stand-alone groups of complex constants and latter allows you to use those constants as choises.

Let’s examine some simple example and define some constants:

from candv import SimpleConstant, Constants

class METHOD_TYPE(Constants):
    Available HTTP methods.
    GET = SimpleConstant()
    PUT = SimpleConstant()
    POST = SimpleConstant()
    DELETE = SimpleConstant()
    TRACE = SimpleConstant()

Here we defined a group of constants with no attributes. Looks pretty, let’s use it:

from candv_x.django.choices import ChoicesField

from django.db import models
from django.utils.translation import ugettext_lazy as _

from . constants import METHOD_TYPE

class Request(models.Model):

    method = ChoicesField(
        help_text=_("Example of simple candv constants"),

That’s all. You can pass some default value if you want, e.g. default=METHOD_TYPE.GET.

Now you can render it:

{% for r in requests %}
  <li>{{ }}</li>
{% endfor %}

The output will contain GET, PUT, POST, etc. Want more? Let’s add values, verbose names and help texts:

from candv import VerboseValueConstant, Values
from django.utils.translation import ugettext_lazy as _

class RESULT_TYPE(Values):
    Possible operation results.
    SUCCESS = VerboseValueConstant(
        help_text=_("Yay! Everything is good!")
    FAILURE = VerboseValueConstant(
        help_text=_("Oops! Something went wrong!")
    PENDING = VerboseValueConstant(
        help_text=_("Still waiting for the task to complete...")

Please, refer to candv usage to learn how to define and use constants. You may find candv customization useful too.

Here we have used Values as container and VerboseValueConstant as class for items. Each constant has a name (e.g. SUCCESS), a value, a verbose text and a help text. All of this you can access directly from everywhere.

Field definition does not differ much from previous:

from candv_x.django.choices import ChoicesField

from django.db import models
from django.utils.translation import ugettext_lazy as _

from . constants import RESULT_TYPE

class Request(models.Model):

    result = ChoicesField(
            help_text=_("Example of complex candv constants with verbose names, "
                        "help texts and inner values"),

You may use blank=True if you wish, there’s no problem. Let’s output our data:

{% for r in requests %}
    <td style="color: #{{ r.result.value }};" title="{{ r.result.help_text }}">
      {{ r.result.verbose_name }}
{% endfor %}

Not so hard, innit?

You can pass any constants to ChoicesField from your old projects or external libraries. Enjoy!


  • Django admin renders choices by converting them to strings. So, __str__ and __unicode__ methods will be automatically overriden for constant items. It will return the name of the constant. By default, constants in candv do not have those methods at all (I cannot find a reason why the should to), so it seems not to be a problem. Just be aware.

  • candv supports creating hierarchies of constants. If you have some reason to use them as choices for DB field, take into accout that choices will be built only from top-level group of constants.

Things to think about

  • Django has MultipleChoiceField and TypedMultipleChoiceField. I haven’t used used them, but I think it can be useful to implement analogues for ‘candv’, especially for MultipleChoiceField.

  • I think, there is a place to think about implementation of full support of hierarchies. Maybe it’s possible to make some nested choices, or at least flatten them.


You can click a version name to see a diff with the previous one.

  • 1.1.5 (Aug 1, 2015)

  • 1.1.4 (Jul 2, 2015)

    • Add support for Python 3 (issue #6).

    • Add support for migrations in Django >= 1.7 (issue #7).

    • Imports which will become deprecated in Django 1.9 are not used now.

  • 1.1.3 (Oct 11, 2014)

    • candv dependency updated up to v1.2.0.

    • Add support for South (issue #4).

    • Choices’ form field can display help text as option’s title now (issue #1).

  • 1.1.0 (Jul 19, 2014)

    • rename package to choices and move into candv_x.django (x stands for extensions)

  • 1.0.0 (Jun 22, 2014)

    Initial version

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