Lazy choices in Django form

Question

I have a Django my_forms.py like this:

class CarSearchForm(forms.Form):  
    # lots of fields like this
    bodystyle = forms.ChoiceField(choices=bodystyle_choices())  

Each choice is e.g. ("Saloon", "Saloon (15 cars)"). So the choices are computed by this function.

def bodystyle_choices():  
    return [(bodystyle.bodystyle_name, '%s (%s cars)' %  
          (bodystyle.bodystyle_name, bodystyle.car_set.count()))  
          for bodystyle in Bodystyle.objects.all()]

My problem is the choices functions are getting executed every time I merely import my_forms.py. I think this is due to the way Django declares its fields: in the class but not in a class method. Which is fine but my views.py imports my_forms.py so the choices lookups are done on every request no matter which view is used.

I thought that maybe putting choices=bodystyle_choices with no bracket would work, but I get:

'function' object is not iterable

Obviously I can use caching and put the "import my_forms" just in the view functions required but that doesn't change the main point: my choices need to be lazy!

Answer 1

You can use the "lazy" function :)

from django.utils.functional import lazy

class CarSearchForm(forms.Form):  
    # lots of fields like this
    bodystyle = forms.ChoiceField(choices=lazy(bodystyle_choices, tuple)())

very nice util function !

Answer 2

Try using a ModelChoiceField instead of a simple ChoiceField. I think you will be able to achieve what you want by tweaking your models a bit. Take a look at the docs for more.

I would also add that ModelChoiceFields are lazy by default :)

Answer 3

Expanding on what Baishampayan Ghose said, this should probably be considered the most direct approach:

from django.forms import ModelChoiceField

class BodystyleChoiceField(ModelChoiceField):
    def label_from_instance(self, obj):
        return '%s (%s cars)' % (obj.bodystyle_name, obj.car_set.count()))

class CarSearchForm(forms.Form):  
    bodystyle = BodystyleChoiceField(queryset=Bodystyle.objects.all())

Docs are here: https://docs.djangoproject.com/en/1.8/ref/forms/fields/#modelchoicefield

This has the benefit that form.cleaned_data['bodystyle'] is a Bodystyle instance instead of a string.

Answer 4

You can now just use (since I think Django 1.8):

class CarSearchForm(forms.Form):  
    # lots of fields like this
    bodystyle = forms.ChoiceField(choices=bodystyle_choices)  

Note the missing parenthesis. If you need to pass arguments, I just make a special version of the function with them hardcoded just for that form.