CODEWITHSUNDEEP
python
Muricula
Muricula

Reputation: 1234

Python os.popen requires an integer?

I'm running python 2.7.3 and doing some basic stuff involving the os module.

import os

def main():
    f= os.popen('cat  > out', 'w',1)
    os.write(f, 'hello pipe')
    os.close(f)

main()

Based on examples I've seen I would expect the code to work, but the interpreter gives this error:

Traceback (most recent call last):
  File "./test.py", line 11, in <module>
    main()
  File "./test.py", line 8, in main
    os.write(f, 'hello pipe')
TypeError: an integer is required

Okay, off to the documentation. The help page says:

write(...)
    write(fd, string) -> byteswritten

    Write a string to a file descriptor.

fd seems to stand for file descriptor. Presumably this is what you get when you do something like:

file = open('test.py')

No surprise, online documentation says the exact same thing. What's going on here?

Upvotes: 1

Views: 951

Answers (1)

Claudiu
Claudiu

Reputation: 229521

No, a "file descriptor" is an integer, not the file object. To go from a file object to a file descroptor, call file.fileno(). To wit:

>>> f = open("tmp.txt", "w")
>>> help(f.fileno)
Help on built-in function fileno:

fileno(...)
    fileno() -> integer "file descriptor".

    This is needed for lower-level file interfaces, such os.read().

>>> f.fileno()
4

Instead of using that, though, you probably just want to do the following, unless you really need to use the low-level functions for some reason:

f = os.popen('cat  > out', 'w',1)
f.write('hello pipe')
f.close()

Upvotes: 7

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