Imagine we\u2019ve launched our feedback form, and the e-mails have started tumbling in. There\u2019s just one problem: some of the submitted messages are just one or two words, which isn\u2019t long enough for us to make sense of. We decide to adopt a new validation policy: four words or more, please.<\/p>\n
There are a number of ways to hook custom validation into a Django form. If our rule is something we will reuse again and again, we can create a custom field type. Most custom validations are one-off affairs, though, and can be tied directly to the Form class. We want additional validation on the message field, so we add a clean_message() method to our Form class:<\/p>\n
from django import forms<\/p>\n
class ContactForm(forms.Form):
\nsubject = forms.CharField(max_length=100)
\nemail = forms.EmailField(required=False)
\nmessage = forms.CharField(widget=forms.Textarea)<\/p>\n
def clean_message(self):
\nmessage = self.cleaned_data[‘message’]\nnum_words = len(message.split())
\nif num_words < 4:
\nraise forms.ValidationError(“Not enough words!”)
\nreturn message<\/p>\n
Django\u2019s form system automatically looks for any method whose name starts with clean_ and ends with the name of a field. If any such method exists, it\u2019s called during validation. Specifically, the clean_message() method will be called after the default validation logic for a given field (in this case, the validation logic for a required CharField). Because the field data has already been partially processed, we pull it out of self.cleaned_data.<\/p>\n
Also, we don\u2019t have to worry about checking that the value exists and is non-empty; that\u2019s done by the default validator. We naively use a combination of len() and split() to count the number of words. If the user has entered too few words, we raise a forms.ValidationError. The string attached to this exception will be displayed to the user as an item in the error list (Figure 6-8).<\/p>\n
<\/p>\n
![\"\"](\"https:\/\/www.vskills.in\/lms\/wp-content\/uploads\/2016\/07\/34.jpg\")
\nIt\u2019s important that we explicitly return the cleaned value for the field at the end of the method. This allows us to modify the value (or convert it to a different Python type) within our custom validation method. If we forget the return statement, then None will be returned, and the original value will be lost.<\/p>\n\n\n