[Python-Dev] file(file) (original) (raw)

A.M. Kuchling amk at amk.ca
Fri Jan 12 18:19:19 CET 2007

Forwarded for discussion from http://www.python.org/sf/1633665.

--amk

forwarded from [http://bugs.debian.org/327060]

Many types in Python are idempotent, so that int(1) works as expected, float(2.34)==2.34, ''.join('hello')=='hello' et cetera.

Why not file()? Currently, file(open(something, 'r')) fails with "TypeError: coercing to Unicode: need string or buffer, file found."

Semantically, file(fd) should be equivalent to os.fdopen(fd.fileno()) or the proposed file.fromfd() (Jp Calderone, Python-dev, 2003). You should get another independent file object that accesses the same file.

What would be gained?

Primarily, it would allow you to derive classes from file more easily. At present, if you want to derive like so, you're class can only work when passed a file name or buffer.

class file_with_caching(file): def init(self, something): file.init(self, something)

def etcetera...

For instance, you have no way of creating a file_with_caching() object from the file descriptors returned from os.fork().

Also, you have no way of taking a file that is already open, and creating a file_with_caching() object from it. So, you can't use classes derived from file() on the standard input or standard output.

This breaks the nice Linux OS-level definition of a file descriptor. At the Linux level, you have a nice uniform interface where all file descriptors are equally good. At the python level, some are better than others. It's a case where Python unnecessarily restricts what you can do.

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