Django custom form field checks that any input email is from a registered user, multiple comma delimitated input accepted

UserEmailField.py

from django import forms
from django.core.exceptions import ValidationError
from django.contrib.auth.models import User


class UserEmailField(forms.EmailField):
    '''
    This field subclasses EmailField, so we get the validation that no blanks are supplied and                that an actual email address was supplied
    This custom field adds validation for the existence of a registered user with this email. 
    '''
    def to_python(self, value):
        #call the super to_python, which returns a unicode object, trim any whitespace left between emails by the user, then create a list of emails
        value = super(UserEmailField, self).to_python(value).strip().replace(' ', '').split(',')
        #remove any 'empty' email, which would be caused by the user leaving a trailing comma
        value = [email for email in value if email != u'']
        return value
    
    def find_user(self, value):
        errorlist = []
        for email in value:
            #value is list returned by to_python
            #if we don't find a user for an email, add it to the list of errors
            try:
                User.objects.get(email=email)
            except:
                errorlist.append(email)
        if errorlist:
            #raise a field error
            raise forms.ValidationError(u"we haven't found a user for the following email(s): '%s'" % ( ', '.join(errorlist)))

    def clean(self, value):
        #run all the default validation, then call find_user
        value = self.to_python(value)
        for email in value:
            #check that each email in the list is a real email
            self.run_validators(email)
        #validate checks that something was imput, or raises 'field is required'    
        self.validate(value)
        #find_user checks that each email supplied belongs to a registered user
        self.find_user(value)
        return value