In the BZ2File object of bz2 module the writelines()
method does not
check its closed state before doing the actual work so
its behavior
it's different from write()'s behavior. See:

>>> from bz2 import BZ2File
>>> f = BZ2File("foo", "w")
>>> f.close()
>>> f.closed
1
>>> f.write("foobar")
Traceback (most recent call last):
 File "<stdin>", line 1, in <module>
ValueError: I/O operation on closed file
>>> f.closed
1
>>> f.writelines(["foobar"])
Traceback (most recent call last):
 File "<stdin>", line 1, in <module>
IOError: unknown IO error

It also doesn't check if it can write for real:

>>> from bz2 import BZ2File
>>> f = BZ2File("foo", "r")
>>> f.write("foobar")
Traceback (most recent call last):
 File "<stdin>", line 1, in <module>
IOError: file is not ready for writing
>>> f.writelines(['foobar'])
Traceback (most recent call last):
 File "<stdin>", line 1, in <module>
RuntimeError: wrong sequence of bz2 library commands used

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user_id=849994

Thanks for the patch, this has been fixed in rev.
51285--51288. (The first example actually segfaulted here!)