Question

I have a parameter that I am passing in to the form. It is a dictionary to configure the form.

How can I set the max_length of a field using a value from the dictionary?

class StepTwoForm(forms.Form):
    number = forms.CharField()

    def __init__(self, *args, **kwargs):
        if 'config' in kwargs:
           config = kwargs.pop('config')
           super(StepTwoForm, self).__init__(*args, **kwargs)
           self.fields['number'].max_length = config['MAX_LENGTH']

I also just tried hardcoding the max_length to an arbitrary value which didn’t work either.

Or am I going about this the wrong way?

Answer 1

That will work, however you need to move the super(..) call outside of the condition, otherwise the form won’t get setup properly.

from django.core import validators

class StepTwoForm(forms.Form):
    number = forms.CharField()

    def __init__(self, *args, **kwargs):
        config = kwargs.pop('config', {})
        super(StepTwoForm, self).__init__(*args, **kwargs)
        if 'MAX_LENGTH' in config:
            validator = validators.MaxLengthValidator(config['MAX_LENGTH'])
            self.fields['number'].validators.append(validator)

UPDATE

It looks like max_length and min_length are used upon initialization, so it’s too late to set the max_length parameter. You need to manually add the validator. I’ve updated the code to reflect the change. Here is the relevant code in Django: https://github.com/django/django/blob/master/django/forms/fields.py#L186-L192