What exactly is "pass"? What should it be?

[Chris Rebert]

On Thu, Nov 17, 2011 at 6:18 PM, John Ladasky <ladasky at my-deja.com> wrote:
> Hi folks,
>
> I'm trying to write tidy, modular code which includes a long-running process.  From time to time I MIGHT like to check in on the progress being made by that long-running process, in various ways.  Other times, I'll just want to let it run.  So I have a section of code which, generally, looks like this:
>
> def _pass(*args):
>    pass
>
> def long_running_process(arg1, arg2, arg_etc, report = _pass):
>    result1 = do_stuff()
>    report(result1)
>    result2 = do_some_different_stuff()
>    report(result2)
>    result3 = do_even_more_stuff()
>    report(result3)
>    return result3
>
> This does what I want.  When I do not specify a report function, the process simply runs.  Typically, when I do supply a report function, it would print something to stdout, or draw an update through a GUI.
>
> But this approach seems a tad cumbersome and unPythonic to me, particularly the part where I define the function _pass() which accepts an arbitrary argument list, and does nothing but... pass.

Seems fine to me (good use of the null object pattern), although I
might define _pass() to instead take exactly 1 argument, since that's
all you ever call report() with in your example.

> This has led me to ask the question, what exactly IS pass?  I played with the interpreter a bit.
>
> IDLE 2.6.6      ==== No Subprocess ====
>>>> pass
>>>> pass()
> SyntaxError: invalid syntax
>>>> type(pass)
> SyntaxError: invalid syntax
>
> So, pass does not appear to be a function, nor even an object.  Is it nothing more than a key word?

Correct:
http://docs.python.org/reference/simple_stmts.html#pass
http://docs.python.org/reference/lexical_analysis.html#keywords

Cheers,
Chris