ExecutionScenarios

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Scenes (scenarios, user stories, …) of program execution with Python.

<insert hangman picture here>

Core Subprocess “features” to watch for:

• child exception propagation if executing Python script (http://docs.python.org/2/library/subprocess.html#exceptions)

• check_call() or check_output() return exception on non-zero exit code (other functions do not)

Fire and Forget

Crossplatform shell automation

I am a user, who tired of writing shell commands by hand, and wants a crossplatform way of executing them on Window/Linux/Mac OS. I don’t care about security - I am my own evil Pinocchio.

1. “i only care that the program is successfully started”
Algorithm:

• execute the command

• get the signal that it is successfully started (no ‘command not found’ error)

• detach and let the process live its own live

subprocess way

subprocess.Popen(command, shell=True)

Problems:

• type of command{.backtick} argument affects crossplatform behavior

  • [ ] here should be a table of shell=True{.backtick}, typeof(command), unix, windows, mac os

• child Python exceptions: affected|unaffected

Libraries on Execution

• http://amoffat.github.com/sh/ - avoids suprocess altogether, linux-only for now

Notes about bad design in subprocess

check_call() or check_output() are shortcuts for executing Popen calls (convenience functions), but they modify the error handling behavior, leading to new exceptions when return codes are not the same. There is a also a call() helper that doesn’t add new exceptions in this case and repeats Popen behaviour. This is what is called “inconsistent design” of API.

The check_* in the names of the helpers is probably because of those added exceptions, but that’s not obvious. Names became non-friendly for humans. There might be a good rational user story behind this change, but it is missing in the context of this API, so as a user, I don’t have a chance to memorize the conditions, when this can be handy. Defining the conditions is the goal of writing this page.