[Courses] [python] Lesson 3: variable scope

[Akkana Peck]

Brad and Monique wanted to know about variable scoping. This is
a relatively advanced topic that will probably only make sense to
folks who have done a fair amount of programming in other languages.
So, beginners, feel free to skip this message -- you won't need it
for anything in this class.

Brad writes:
> count = 0 # Initialize the count variable and set it to zero
[ ... ]
> for i in wordlist :
>     count += 1
> 
> Quick question on that one, from one of those Perl slobs (ie, me) --
> I stuck in an initialization value for variable "count" because it
> felt awkward not to have anything -- I'm used to strict variable
> declarations in Perl (which yes, I know you technically don't have
> to use, but you'd be a dope not to). Is there an equivalent here?

Python doesn't have any way of formally declaring variables, or
restricting them to certain types (like saying i will always be an
integer). You create a variable by setting it, like i = 42, and
you can do that any time.

But in your example, you did need the initialization, because you
wouldn't be able to say "count += 1" if you hadn't already set count;
count += 1 means "take the current value and add 1 to it", so you'd
get a "not defined" error if you hadn't set count first. 

> And if not, how do you determine variable scope, or does such a
> concept even exist within Python?
> 
> I guess I might be getting ahead of things a bit.

For new programmers, variable scope basically means "if I create a
variable i inside my program, where in the program can I use it?"

Basically, you can use a variable any time after you first set it.
But when you throw in functions the scoping gets a bit complicated.
(I'll be covering functions in lesson 4, anyone who's not familiar
with them yet.)

Anyway, if you have a function, and you've set a variable i before
you call the function, then the function will see the variable by
that name -- but only for reading. If it ever tries to set it, you'll
end up with a local variable i which is not connected to the global
variable i.  If you want the function to be able to set the global
i, there's a "global" keyword you can use inside the function. 

As I said, it's confusing, and I've never found a good write-up on
it -- a lot of Python books seem to gloss over scoping entirely, and
most of what I just wrote, I determined empirically by trying it,
IMHO, it's one of the language's weakest points. Happily, you can
mostly avoid it by using objects and functions instead of globals.

	...Akkana