On 7/13/07, J. J. Ramsey <[EMAIL PROTECTED]> wrote: > In Perl, there is a module called "Tie::File". What it does is tie a > list to each line of a file. Change the list, and the file is > automatically changed, and on top of this, only the bits of the file > that need to be changed are written to disk. At least, that's the > general idea. > > I was wondering if something roughly similar could be done in Python, > or at the very least, if I can avoid doing what amounts to reading the > whole file into memory, changing the copy in memory, and writing it > all out again.
The mechanism behind Perl's ties -- an array that, when read from or written to, passes control to a user function -- is easy to implement in Python. See the documentation of the mapping protocol <http://docs.python.org/ref/sequence-types.html>: >>> class SpecialList(object): ... def __getitem__(self, index): ... return "Line %d" % index >>> foo = SpecialList() >>> foo[42] 'Line 42' >From the documentation for Tie::File, it doesn't look like a trivial piece of code; for example, it has to maintain a table in memory containing the offset of each newline character it's seen for fast seeking, and it has to handle moving large chunks of the file if the length of a line changes. All this could be implemented in Python, but I don't know of a ready-made version off the top of my head. If all you want is to read the file line-by-line without having the whole thing in memory at once, you can do " -- http://mail.python.org/mailman/listinfo/python-list
