Question

I have a Django app with custom form fields, some of which have slow operations in their constructors. I was surprised recently to find out that those constructors were getting called when Django itself was starting up, even before a user does something that requires that form in a view.

Why are they getting instantiated at server start?

Example:

urls.py:

from myapp.views import view1
...
url(r'^test$', view1.test),

views/view1.py:

class MyForm(ModelForm):
    class Meta:
        model = MyModel
    field1 = MyChoiceField()

class MyChoiceField(ChoiceField):
    def __init__(self, choices=(), required=True, widget=None, label=None,
             initial=None, help_text=None, *args, **kwargs):
    super(ChoiceField, self).__init__(required, widget, label, initial,
                                      help_text, *args, **kwargs)     
    self.choices = [(m.id, m.name) for m in ReallyLargeTableModel.objects.all()]     

If I set a break point inside that field constructor, then start up Django, it breaks the first time I request any page, even if the view in question does not need that form or field. The stacktrace leads back to the import line in urls.py.

Is this because I’m importing view1 in urls.py, instead of importing view1.test?

Edit: This isn’t Django specific, here is a test case the illustrates the behavior:

class Something():
    def __init__(self):
        print "Something __init__() called"

class UsesSomething():
    field = Something()

If you run this in the interactive terminal, it will print “Something init() called”. This was surprising to me because I have not actually instantiated a UsesSomething object.

Answer 1

Because you instantiate the fields in the form definition, which is presumably being imported by one of your views.

The field init is the wrong place to do this sort of dynamic initialization, for this exact reason. You want something that is called when the form is initialized: ie, the form’s __init__.

That said, you don’t actually want to do this at all – you just need to use forms.ModelChoiceField, which takes a queryset and does the dynamic assignment of choices for you.

class MyForm(ModelForm):
    field1 = forms.ModelChoiceField(queryset=ReallyLargeTableModel.objects.all())