django - how to process/clean a field before validation

I think the process of form validation (and its correct order) is really well documented. http://docs.djangoproject.com/en/dev/ref/forms/validation/#form-and-field-validation

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

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

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)