open（）不会在for循环中打开文件描述符

Question

I'm trying to create a for loop and open bunch of file descriptios, but unfortunately it doesn't work for me in the for loop.

>>> import os

>>> os.getpid()
6992

>>> open('/tmp/aaa', 'w')
<open file '/tmp/aaa', mode 'w' at 0x7fa7c9645ae0>

>>> for i in xrange(10):
...     open('/tmp/aaa{0}'.format(str(i)), 'w')
...

Above script only opens 1 fd:

vagrant@workspace:~$ ls -alht /proc/6992/fd/ | grep tmp
l-wx------ 1 vagrant vagrant 64 Nov 29 15:18 11 -> /tmp/aaa

Questions:

1 - how to open multiple file descriptors using for loop?

2 - what's wrong with above code?

Answer 1

open returns a file object that you operate on.

If you discard the return value, Python will garbage collect it, thus closing the file. So, calling open in a loop without using the return value is useless as all the file objects returned will be garbage collected.

What you want is to save those file objects:

Files = [open('/tmp/aaa{0}'.format(str(i)), 'w') for i in xrange(10)]

Some tips:

1. str(i) isn't needed when you use format as the latter cares about type conversions itself.
2. You don't need the zero in {0} in the format string as well because Python inserts format s i -th argument (count starts after the format string) instead of the i -th pair of braces in the format string.

All in all, the code could be simplified to:

Files = [open('/tmp/aaa{}'.format(i), 'w') for i in xrange(10)]

Answer 2

You need to get the file descriptor, and after you do whatever you want to do with it, close it to have it flushed.

so you need something more like this:

for i in range(10):
    f = open('/tmp/aaa{0}'.format(str(i)), 'w')
    # manipulate the file
    f.close()

If you need to save those file descriptors to access them later in the code, you should save them somewhere, like a list.

fs = [open('/tmp/aaa{0}'.format(str(i)), 'w') for i in range(10)]
# manipulate files
_ = [f.close() for f in fs]