The compressor/decompressor classes from bz2 and lzma modules rely on __init__() for initialization, but it is not guaranteed to be called. Method calls on an uninitialized object crash:

>>> from bz2 import BZ2Compressor as C
>>> c = C.__new__(C)
>>> c.compress(b'')
Segmentation fault (core dumped)

I see two ways to fix this:

1) Move some initialization (notably, for "lock" field) to __new__() and add initialization checks to other methods. This should be backwards-compatible.

2) Move all initialization to __new__(). Since compressor/decompressor classes are not subclassable, it'll break only code than repeatedly calls __init__() on the same object. The simplicity of the fix might outweigh the necessity to support such code.
(However, in 2.7, classes in bz2 *are* subclassable; lzma is not present in 2.7).

Which way is more preferable?