Python - How do I convert "an OS-level handle to an open file" to a file object?

Question

tempfile.mkstemp() returns:

a tuple containing an OS-level handle to an open file (as would be returned by os.open()) and the absolute pathname of that file, in that order.

How do I convert that OS-level handle to a file object?

The documentation for os.open() states:

To wrap a file descriptor in a "file object", use fdopen().

So I tried:

>>> import tempfile
>>> tup = tempfile.mkstemp()
>>> import os
>>> f = os.fdopen(tup[0])
>>> f.write('foo\n')
Traceback (most recent call last):
  File "<stdin>", line 1, in ?
IOError: [Errno 9] Bad file descriptor

Prerequisite

Remember to mark an answer as "Accepted" if it worked for you. – Wounded

Answer 1

You can use

os.write(tup[0], "foo\n")

to write to the handle.

If you want to open the handle for writing you need to add the "w" mode

f = os.fdopen(tup[0], "w")
f.write("foo")

Snooze

That works--thanks. But technically fdopen returns a file object (and you pass in a file descriptor), so if I could edit your answer I'd change it to "f = os.fdopen(tup[0], "w");f.write("foo") – Prerequisite

If you don't use os.fdopen, you need to make sure you close the temp file handle with os.close(tup[0]) according to logilab.org/blogentry/17873. – Manny

Answer 2

Here's how to do it using a with statement:

from __future__ import with_statement
from contextlib import closing
fd, filepath = tempfile.mkstemp()
with closing(os.fdopen(fd, 'w')) as tf:
    tf.write('foo\n')

Prerequisite

Is closing really necessary? os.fdopen returns a TextIOWrapper whose __exit__ turned out to close it, tested on Python 3.5 (CPython). – Towne

Answer 3

You forgot to specify the open mode ('w') in fdopen(). The default is 'r', causing the write() call to fail.

I think mkstemp() creates the file for reading only. Calling fdopen with 'w' probably reopens it for writing (you can reopen the file created by mkstemp).

Compagnie

a temporary (hence empty) file ready only for reading would be useless. mkstemp returns the file opened in binary mode for both reading and writing, per the docstring. – Five

Answer 4

temp = tempfile.NamedTemporaryFile(delete=False)
temp.file.write('foo\n')
temp.close()

Endomorphism

The "delete" parameter was added in version 2.6, so that won't work for older Python versions. – Affirmative

Answer 5

What's your goal, here? Is tempfile.TemporaryFile inappropriate for your purposes?

Seifert

I don't want the file destroyed as soon as it's closed. (And I want to be sure the file to be visible.) – Prerequisite

then you can pass delete=False to NamedTemporaryFile – Endomorphism

Answer 6

I can't comment on the answers, so I will post my comment here:

To create a temporary file for write access you can use tempfile.mkstemp and specify "w" as the last parameter, like:

f = tempfile.mkstemp("", "", "", "w") # first three params are 'suffix, 'prefix', 'dir'...
os.write(f[0], "write something")

Carreno