python - Best practice: update/extend subclass class attribute -

this little curiosity. find myself updating class attribute inherited in subclass , i'd know how others deals it. know can't update them in __init__: since it's class attribute update superclass instances well. this:

class a(object):     attribute = {         1: 'a',         ...,         5: 'e',     }  class b(a):     attribute = {         1: 'a',         ...,         5: 'e',         6: 'f',     } 

but, since follow dry principle as possible , i'd know if use more elegant way without copy , paste attribute.

as asked concrete example django forms:

class loginform(authenticationform):      username = forms.emailfield(label=_('email'))      error_messages = {         'inactive': _('this account inactive.'),         'invalid_login': _('please enter correct username , password. '                            'note both fields may case-sensitive.'),         'unconfirmed': _('your account not confirmed yet. please '                          'check email , follow instructions '                          'in order activate it.'),     }      def confirm_login_allowed(self, user):         super(loginform, self).confirm_login_allowed(user)         if not user.confirmed:             raise forms.validationerror(                 self.error_messages['unconfirmed'],                 code='unconfirmed') 

the error_messages attribute inherited django.contrib.auth.forms.authenticationform wanted add 'unconfirmed' key had copy , paste 'inactive' , 'invalid_login' keys too.

if put in __init__ every instance of subclass (b in case), see update. time not if specify class directly:

b.attribute[6] # keyerror 

one way without using __init__ to:

class b(a):     attribute = a.attribute.copy()     attribute.update({           6: 'f',           7: 'g',           })  print(b().attribute) # --> {1:'a', ..., 5:'e', 6:'f', 7:'g'} print(a().attribute) # --> {1:'a', ..., 5:'e'} 

the point remember class's code executed created, can use normal python make adjustments.