Question

Possible Duplicate:
Assignment Condition in Python While Loop

What’s the equivalent of this:

while (line = p.stdout.readline()) != '':
    ...

in Python?

I don’t like having to do this:

line = p.stdout.readline()
while line != '':
    sys.stdout.write(line)
    line = p.stdout.readline()

Although I’ve been using the latter for years… I suspect there is no alternative. I thought p.stdout supported iteration, like

for line in p.stdout:
    sys.stdout.write(line)

but unfortunately it doesn’t, unlike the handle returned from open() for example.

Edit: Sorry, I was wrong, it does support it, the problem is that I can’t get it out right away like I can with p.stdout.readline(). Adding sys.stdout.flush() doesn’t seem to help, so it must be getting buffered inside p.stdout.

Answer 1

You can accomplish this using the built-in function iter() using the two-argument call method:

for line in iter(p.stdout.readline, ''):
    ...

Documentation for this:

iter(o[, sentinel])
…
If the second argument, sentinel, is given, then o must be a callable object. The iterator created in this case will call o with no arguments for each call to its next() method; if the value returned is equal to sentinel, StopIteration will be raised, otherwise the value will be returned.

One useful application of the second form of iter() is to read lines of a file until a certain line is reached. The following example reads a file until the readline() method returns an empty string:

with open('mydata.txt') as fp:
    for line in iter(fp.readline, ''):
        process_line(line)

edit: Based on your edit it looks like your actual problem is related to buffering. Based on the context I am guessing you are using subprocess.Popen() with stdout=subprocess.PIPE. Instead of using the file handle p.stdout directly you should be p.communicate() to read the output of your subprocess.