- an OS-level handle to an open file (as would be returned by os.open())
- the absolute pathname of that file.
I often see code using mkstemp only to get the filename to the temporary file, following a pattern such as:
from tempfile import mkstemp import os def need_temp_storage(): _, temp_path = mkstemp() os.system('some_commande --output %s' % temp_path) file = open(temp_path, 'r') data = file.read() file.close() os.remove(temp_path) return data
This seems to be working fine, but there is a bug hiding in there. The bug will show up on Linux if you call this functions many time in a long running process, and on the first call on Windows. We have leaked a file descriptor.
The first element of the tuple returned by mkstemp is typically an integer used to refer to a file by the OS. In Python, not closing a file is usually no big deal because the garbage collector will ultimately close the file for you, but here we are not dealing with file objects, but with OS-level handles. The interpreter sees an integer and has no way of knowing that the integer is connected to a file. On Linux, calling the above function repeatedly will eventually exhaust the available file descriptors. The program will stop with:
IOError: [Errno 24] Too many open files: '/tmp/tmpJ6g4Ke'
On Windows, it is not possible to remove a file which is still opened by another process, and you will get:
Windows Error [Error 32]
Fixing the above function requires closing the file descriptor using os.close_():
from tempfile import mkstemp import os def need_temp_storage(): fd, temp_path = mkstemp() os.system('some_commande --output %s' % temp_path) file = open(temp_path, 'r') data = file.read() file.close() os.close(fd) os.remove(temp_path) return data
If you need your process to write directly in the temporary file, you don't need to call os.write_(fd, data). The function os.fdopen_(fd) will return a Python file object using the same file descriptor. Closing that file object will close the OS-level file descriptor.