As an experiment with the new IO library (http://docs.google.com/Doc?
id=dfksfvqd_1cn5g5m), I added support for walking through the lines
of a file backwards using the reversed() iterator:
for line in reversed(open(path)):
...
It works by scanning backwards through the file a block at a time
(using seek()) but will handle arbitrary length lines.
The patch is at http://www.python.org/sf/1677872. The code is
currently directly in Lib/io.py, but in fact the only internal access
it needs is adding the __reversed__ hook to TextIOWrapper.
It's useful for scanning backwards through large log files, but it's
also IMHO a nice example of the benefits of the new library. The
version of this that I used under python 2.5 had to use os.seek() etc
on file descriptors, whereas now it just wraps a new buffering class
(io.TextLineReverser) around the raw IO object. Among other things
it makes unit tests simpler - instead of messing around with
temporary files the tests can do things like:
b = io.BytesIO(b'one\ntwo\nthree\n')
assert list(io.TextLineReverser(b)) == [ 'three\n', 'two\n', 'one
\n' ]
Mark Russell
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