Automatically responding to program output


Command-line programs tend to be designed for interactive shells, which frequently manifests as waiting around for user input, or “prompts”. Well-designed programs offer options for pre-empting such prompts, resulting in an easily automated workflow – but with the rest, interactivity is unavoidable.

Thankfully, Invoke’s Runner class not only forwards your standard input to the running program (allowing you to manually respond to prompts) but it can also be configured to respond automatically on your behalf.

Basic use

The mechanism for this automation is the watchers kwarg to the method (and its wrappers elsewhere, such as and, which is a list of StreamWatcher-subclass instances configured to watch for patterns & respond accordingly. The simplest of these is Responder, which just replies with its configured response every time its pattern is seen; others can be found in the watchers module.


As with all other arguments to run, you can also set the default set of watchers globally via configuration files.

Take for example this program which expects a manual response to a yes/no prompt:

$ excitable-program
When you give the OK, I'm going to do the things. All of them!!
Are you ready? [Y/n] y
OK! I just did all sorts of neat stuff. You're welcome! Bye!

You could call run("excitable-program"), manually watch for the prompt, and mash Y by hand. But if you instead supply a Responder like so:

def always_ready(c):
    responder = Responder(
        pattern=r"Are you ready? \[Y/n\] ",
    )"excitable-program", watchers=[responder])

Then Runner passes the program’s stdout and stderr through responder, which watches for "Are you ready? [Y/n] " and automatically writes y (plus \n to simulate hitting Enter/Return) to the program’s stdin.


The pattern argument to Responder is treated as a regular expression, requiring more care (note how we had to escape our square-brackets in the above example) but providing more power as well.