Python write in mkstemp() file

Question

I am creating a tmp file by using :

from tempfile import mkstemp

I am trying to write in this file :

tmp_file = mkstemp()
file = open(tmp_file, 'w')
file.write('TEST\n')

Indeed I close the file and do it proper but when I try to cat the tmp file, it stills empty..It looks basic but I don't know why it doesn't work, any explanations ?

Answer 1

mkstemp() returns a tuple with a file descriptor and a path. I think the issue is that you're writing to the wrong path. (You're writing to a path like '(5, "/some/path")'.) Your code should look something like this:

from tempfile import mkstemp

fd, path = mkstemp()

# use a context manager to open the file at that path and close it again
with open(path, 'w') as f:
    f.write('TEST\n')

# close the file descriptor
os.close(fd)

Answer 2

mkstemp returns (fd, name) where fd is an os-level file descriptor ready for writing in binary mode; so all you need is to use os.write(fd, 'TEST\n'), and then os.close(fd).

No need to re-open the file using either open or os.fdopen.

jcomeau@bendergift:~$ python
Python 2.7.16 (default, Apr  6 2019, 01:42:57) 
[GCC 8.3.0] on linux2
Type "help", "copyright", "credits" or "license" for more information.
>>> import os
>>> from tempfile import mkstemp
>>> fd, name = mkstemp()
>>> os.write(fd, 'TEST\n')
5
>>> print(name)
/tmp/tmpfUDArK
>>> os.close(fd)
>>> 
jcomeau@bendergift:~$ cat /tmp/tmpfUDArK 
TEST

In command-line testing, of course, there is no need to use os.close, since the file is closed on exit anyway. But that is bad programming practice.

Answer 3

This example opens the Python file descriptor with os.fdopen to write cool stuff, then close it (at the end of the with context block). Other non-Python processes can use the file. And at the end, the file is deleted.

import os
from tempfile import mkstemp

fd, path = mkstemp(text=True)

with os.fdopen(fd, 'w') as fp:
    fp.write('cool stuff\n')

# Do something else with the file, e.g.
# os.system('cat ' + path)

# Delete the file
os.remove(path)

Answer 4

The answer by smarx opens the file by specifying path. It is, however, easier to specify fd instead. In that case the context manager closes the file descriptor automatically:

import os
from tempfile import mkstemp

fd, path = mkstemp()

# use a context manager to open (and close) file descriptor fd (which points to path)
with os.fdopen(fd, 'w') as f:
    f.write('TEST\n')

# This causes the file descriptor to be closed automatically