Python os.popen requires an integer?

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?