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I am sorry if it's very basic or already asked before (I googled but couldn't find a simple & satisfactory explanation).

I want to know what sys.stdin.fileno() is?

I saw it in a code and didn't understand what it does. Here's the actual code block,

fileno = sys.stdin.fileno()

if fileno is not None:

new_stdin = os.fdopen(os.dup(fileno))

I just executed print sys.stdin.fileno() in my python command line and it returned 0.

I also searched google, and this (nullage.com)  is the reference I could find, but it also only says,

fileno() -> integer "file descriptor".

This is needed for lower-level file interfaces, such os.read().

So, what exactly does it mean?

python  file-access

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up vote6down voteaccepted

File descriptor is a low-level concept, it's an integer that represents an open file. Each open file is given a unique file descriptor.

In Unix, by convention, the three file descriptors 0, 1, and 2 represent standard input, standard output, and standard error, respectively.

>>> import sys

>>> sys.stdin.fileno()

0

>>> sys.stdout.fileno()

1

>>> sys.stderr.fileno()

2