Raise an exception on --help and --version about docopt

[keleshev]

I'm ok with this change, if you provide the code.

I suggest to modify DocoptExit to be able to select the exit code and whether to write to stderr or stdout (likely to depend on exit code).

Turns out this is actually impossible; SystemExit's ability to terminate the Python interpreter without printing a stack trace, as well as how it determines the exit code to use, are all hardcoded details of the Python implementation itself. For my purposes, however, I can temporarily redirect sys.stdout to a cStringIO instance; as docopt.extras() is the only function in the module that can raise a vanilla SystemExit, catching it and calling cstringio_instance.getvalue() is pretty much equivalent to what I was hoping to accomplish with a custom exception.

[keleshev]

Actually raise SystemExit(42) will exit the program with code 42. So that is possible. The hard part (or, even, maybe, impossible part) is to print something to stderr while exiting using this exception.

[keleshev]

The following script:

import sys


class MyExit(SystemExit):

    def __init__(self, msg):
        self.message = msg
        sys.stderr.write(self.message)
        SystemExit.__init__(self, 2)


raise MyExit('bye')

returns exit status 2 and prints "bye" to stderr. Although it is not exactly what we want. We want to print after the exception was uncaught.

Exactly. What I tried to communicate with "how it determines the exit code" was how, if SystemExit (or sys.exit) is raised with an integer and not caught, it uses the given integer as the exit status; if it is raised with anything else, it exits with status 1 and prints the given argument to stderr. In CPython, this print-to-stderr-if-uncaught logic is written directly in pythonrun.c (more relevantly, in the function handle_system_exit on line 1077.)

[keleshev]

You're right. Looks like a language limitation—they should have not mixed exit code and error message.

Also, I wanted docopt to return code 2 instead of 1 in case of DocoptExit (looks like 2 is more conventional for input/user errors in Unix), but that makes it a bit harder.