gistfile1.txt

(txaws) exarkun@baryon:~/Work/LeastAuthority/txaws$ cat /tmp/frozenwhat.py 
import attr

@attr.s(frozen=True)
class Base(object):
    a = attr.ib()

@attr.s
class Derived(Base):
    b = attr.ib()
    

Derived("a", "b")
(txaws) exarkun@baryon:~/Work/LeastAuthority/txaws$ python /tmp/frozenwhat.py 
Traceback (most recent call last):
  File "/tmp/frozenwhat.py", line 12, in <module>
    Derived("a", "b")
  File "<attrs generated init ceb0d1d1d54ce44d01f6dd968d2fb57e3b4d0016>", line 2, in __init__
  File "/home/exarkun/Environments/txaws/local/lib/python2.7/site-packages/attr/_make.py", line 201, in _frozen_setattrs
    raise FrozenInstanceError()
attr.exceptions.FrozenInstanceError
(txaws) exarkun@baryon:~/Work/LeastAuthority/txaws$