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This doesn't quite work (yet): you've broken the support for
writing binary data to the file via file.write(). Encodings
should only be used for non-binary files.

Also note that you are not freeing the memory allocated by
the "et#" parser for s.

Please add some test cases where you open a binary file and
write:
a) binary strings 
b) contents of a buffer object
c) Unicode objects 
to it.

Case c) should raise an exception. a) and b) should result
in the data being written as-is - without doing any recoding.