forms - Django - Changing widget on GET request according to a choice field -

i want change field widget readonly if choice of form 'yes'. code working fine initiate widget state readonly:

form:

def __init__(self, *args, **kwargs):      super(accountupdate, self).__init__(*args, **kwargs)     self.fields['year'].widget.attrs['readonly'] = u'readonly' 

but how according choice field on request?

model:

class accountupdate(models.model):         updateaddress = models.charfield(max_length=1, null=true, blank=true, choices=option_choices, default='n')     address = models.charfield('your address?', max_length=50)     year = models.charfield(max_length=4) 

choices:

option_choices = (       ('s', 'yes'),       ('n', 'no'),  )