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Tips: for Initial arg of Django's Form fields

Tips: for Initial arg of Django’s Form fields

Django’s Form fields take a initial argument to specify the initial value for that Field.

For more detail about initial, check out the doc Form fields #initial

Basically…

As written there, if you want to set a initial value as a result of a callable, you should pass the callable directly to the argument. like this:

class DateTimeForm(forms.Form):
     now = forms.DateTimeField(initial=datetime.datetime.now)

With arguments

Mistake

Let’s consider the case passing some argument to callable to pass to initial. A common mistake is like this:

# Don't do this
class DateTimeForm(forms.Form):
     now = forms.DateTimeField(initial=datetime.datetime.now(tz))

In this case, the now value would have been fixed at the time you start the server. You should pass a callabse directary, how do you pass arguments to the callable?

Elegant solution

Let’s put together the calling with partial function application. The following is a simple example using functools.partial.

>>> from functools import partial
>>>
>>> zero_until_ten = partial(range, 10)
>>> zero_until_ten()
[0, 1, 2, 3, 4, 5, 6, 7, 8, 9]

Yes, you made a callable without arguments. This technique can be used in the previous example, like this:

class DateTimeForm(forms.Form):
     now = forms.DateTimeField(initial=partial(datetime.datetime.now, tz))

The initial will be passed a callable, so the resulting value will not be fixed.

It’s beautiful.

Inelegant solution

Alternatively, you can write like this:

class DateTimeForm(forms.Form):
     now = forms.DateTimeField()

    def __init__(self, *args, **kwargs):
        super(DateTimeForm, self).__init__(*args, **kwargs)
        self.fields.get('now')._set_choices(datetime.datetime.now(tz))

The beauty declarative was lost.

Another develoaper must read __init__, and you should be careful not to forget the calling super.

Not recommended. Use the functools.partial function.