[Django]-Django – how to process/clean a field before validation

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46👍

✅

I think the process of form validation (and its correct order) is really well documented.

Would you mind sharing your code? There is a clean method on every formfield, responsible for running to_python, validate and run_validators (in this order). to_python accepts the raw value of the widget, coercing it into a python type, validate takes the coerced value and runs field-specific validation.

Answer updated with regards to the code given

clean_message is called after all the other validation has run like to_python, validate and, most importantly, run_validators. I think the last method will check the that the max_length constraint isn’t violated. So you changing the data afterwards has no affect.

The solution is to raise a ValidationError here. Removing max_length will avoid any validation on the input’s length.

class ContactForm(forms.Form):
    message = forms.CharField(widget=forms.Textarea)

    def clean_message(self):
        message = self.cleaned_data['message']
        message = message.replace('a', '') # remove all "a"s from message
        if len(message) >= 5:
            raise ValidationError('Too many characters ...')
        return message
👤Reiner Gerecke

2👍

As an extra, when is_valid() return False, you can check why, printing self.errors inside the form.

if you watch the is_valid() code, this checks the form is_bound and the self.errors is empty

👤Nicolas Rojo

0👍

You can overwrite CharField:

class CustomCharField(forms.CharField):
    def clean(self, value):
        return super().clean(value.replace('a', ''))

Then you can use it in form

class ContactForm(forms.Form):
    message = CustomCharField(widget=forms.Textarea)
👤Qwarky

-1👍

Just adding to Reiner Gerecke.

When the is_valid() method is called on the form, the MultiEmailField.clean() method will be run as part of the cleaning process and it will, in turn, call the custom to_python() and validate() methods.
Just read the “Form field default cleaning” part and its example

👤karan

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