On 11/29/2011 03:06 PM, Colin Higwell wrote:
Hi,
I am just starting to learn Python (I have been at it only a few hours),
so please bear with me. I have a few very small scripts (do you call them
scripts or programs?) which work properly, and produce the results
intended.
However, they are monolithic in nature; i.e. they begin at the beginning
and finish at the end. Having done a little reading, I note that it seems
to be quite common to have a function main() at the start (which in turn
calls other functions as appropriate), and then to call main() to do the
work.
Is that standard best practice?
Thanks
Welcome to Python, and to the comp.lang.python list. Is this your first
experience programming?
Yes, factoring your code from "monolithic" to "modular' (several
functions, or even functions and classes), is good practice.
That's not to say that some problems don't deserve a monolithic answer,
but if you're a beginner, i'd like to see you get into a modular habit.
You can use the words script and program pretty much interchangeably.
in some contexts, each has additional connotations. For example,
somebody used to a compiled language may refer to a python source file
as a script, implying it's not as sophisticated as his own product.
But, closer to home, we usually refer to the file you directly pass to
the interpreter as a script, and any added files that get imported, as
modules, or libraries.
Other times, people will refer to a simple program as a script, implying
that all it does is invoke some standard library functions, or even run
some external programs. But when it gets more complex, it gradually
turns into a "real program."
Why break up a monolith?
Several reasons. If you factor the code into independent functions, and
give them good names, then each piece of the program is easier to
understand. You will especially appreciate that if you come back to the
code after doing something else for two weeks. Similarly if somebody
else has to take over your code, or maybe adapt it to a slightly
different purpose.
Next, if it doesn't quite work, you can exercise the individual pieces
independently, and narrow down the problem more quickly.
Next, if the progfram is slow, usually you can narrow it down to a few
key functions that take most of the time. You can write two versions of
the same function, and do some careful timings to decide which one to use.
Next, some of those functions may be useful in the next program you
write. If you "reuse" the code by copy & paste, and find a problem in
the new one, it's quite possible that the same problem may exist in your
first program. it's easier to bring those changes back if they're in a
function than if they're in lines 14 through 71.
Finally, some of that reusable code may be worth moving to an external
file, called a module. Then the same copy can be literally shared
between multiple projects. This is how libraries were born, and you can
write your own, eventually.
there are many other reasons, but some of them might not make sense to
you yet.
How do you break it up?
First, separate the classic parts that most scripts will have. Put the
imports at the top, along with a comment describing the whole progfram's
purpose. Next put the global variables, which should be few. If there
are any constants, use all UPPERCASE for their names.
Next, put the function and class definitions. Notice that none of them
will be called yet, so the order of execution isn't important to the
compiler. Each function needs a name, and you should use a name that
makes sense to you. Try to write functions that work at a single level
of complexity, and do one complete operation. Try not to do
input/output in the same functions that do the computation.
And finally, put the actual mainline. It could be as simple as a call
to main(), but it may make more sense to you to put the calls to
argument processing here, rather than in a main function. By arguments
here, i'm referring to the stuff you typed on the command line when youi
invoked the script. This part of the code is where you do the magic
incantation:
if __name__ == "__main__":
main()
When the number of functions gets unwieldy, it's time to move some of
them to a new file. They should be related, and should work at the same
level of complexity. And the file name should remind you of their
purpose. At that point, you add an import of that file to your main
source script. Congratulations, you've created a module.
One catch with writing a lengthy reply is that others have already given
you good feedback. Hopefully, mine will complement theirs.
--
DaveA
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