On 11/17/2011 6:45 PM, Chris Rebert wrote:
On Thu, Nov 17, 2011 at 6:18 PM, John Ladasky<lada...@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?
It is a keyword that can appear in a position where a statement is
required by the grammar but there is nothing to do. For example if ..
then .. else .. where nothing happens in the else condition is effectively:
if <condition>:
<then-part>
else:
pass
Bourne shell has a similar construct with the colon statement :
Another python example is where you need to catch an exception (or all
exceptions but don't actually care about what they are)
try:
<european swallows>
except:
pass
Correct:
http://docs.python.org/reference/simple_stmts.html#pass
http://docs.python.org/reference/lexical_analysis.html#keywords
Cheers,
Chris
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