Change error text in typesystem

I’d like to know how redefine error text in Typesytem.

I know I can redefine the errors dict, but I only want change one thing (pattern).

Shorter, i’d like to to this :

class bla (typesystem.String):
pattern = "^complexe pattern$"
errors[‘pattern’] : "another text


i answer to myself, not very hard but not nice…

class bla(typesytem.String):
errors = typesystem.String.errors.copy()
errors[‘pattern’] = “my nex error text”