CODEWITHSUNDEEP
pythonlinux
Liao Zhuodi
Liao Zhuodi

Reputation: 3293

what is the os.close(3) for?

What is the os.close(3) for?

I am reading the python cookbook 2nd chapter 2.9, it explains how the python zip file work. There is one snippet of code in it I don't really got it. 

import zipfile, tempfile, os, sys
handle, filename = tempfile.mkstemp('.zip')
os.close(handle)  # <- handle is int 3 here
z = zipfile.ZipFile(filename, 'w')
z.writestr('hello.py', 'def f(): return "hello world from "+__file__\n')
z.close()
sys.path.insert(0, filename)
import hello
print hello.f()
os.unlink(filename)

os.close() explaination in python docs:

This function is intended for low-level I/O and must be applied to a file descriptor as returned by os.open() or pipe(). To close a “file object” returned by the built-in function open() or by popen() or fdopen(), use its close() method.

The file descriptor in linux from 0,1 & 2 are stdin, stdout & stderror, I don't get what the fd 3 for? Even though I have read this "What is the file descriptor 3 assigned by default? ".

I comment the os.close(handle) out, but the output make no different.

Upvotes: 1

Views: 1808

Answers (2)

davejagoda
davejagoda

Reputation: 2528

You are getting file descriptor 3 because in this case it is the next available file descriptor. As you mentioned, stdin (0), stdout (1) and stderr (2) are automatically opened for you. The link you cited ( https://unix.stackexchange.com/questions/41421/what-is-the-file-descriptor-3-assigned-by-default ) points this out, too.

Upvotes: 0

Charles Duffy
Charles Duffy

Reputation: 295629

Even though Python mostly deals in "file objects", these are an abstraction around OS-level file handles; when actually reading or writing content to a file (or network stream, or other file-like object) at the operating system level, one passes the OS the handle number associated with the file with which one wants to interact. Thus, every file object in Python that's actually backed by an OS-level file handle has such a file descriptor number associated.

File handles are stored in a table, each associated with an integer. On Linux, you can look at the directory /proc/self/fds (substituting a PID number for self to look at a different process) to see which handles have which numbers for a given process.

handle, filename = tempfile.mkstemp('.zip'); os.close(handle), thus, closes the OS-level file handle which was returned to you by mkstemp.

By the way: It's important to note that there's absolutely nothing special about the number 3, and that there is no default or conventional behavior for same implemented at the operating system level; it just happened to be the next available position in the file handle table when mkstemp was called (or, to be more precise, when the C standard library implementation of mkstemp called the OS-level syscall open).

Upvotes: 4

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