Question

I am learning Python and am reading through an example script that includes some variable definitions that look like:

output,_ = call_command('git status')
output,_ = call_command('pwd')

def call_command(command):
    process = subprocess.Popen(command.split(' '),
        stdout=subprocess.PIPE,
        stderr=subprocess.PIPE)
    return process.communicate()

If I print output I get the resulting shell output strung together, so I know it’s concatenating the variables. But I can’t find any reference to the ,_ convention in any of the docs. Can someone explain it to me so that I know for sure I am using it correctly?

Answer 1

The general form

a, b = x, y

is tuple assignment. The corresponding parts are assigned, so the above is equivalent to:

a = x
b = y

In your case, call_command() returns a tuple of two elements (which is what process.communicate() returns). You’re assigning the first one to output and the second one to _ (which is actually a variable name, typically used to name something when you don’t care about the value).