[Tutor] advice on global variables

[Alan Gauld]

On 10/07/12 20:11, Chris Hare wrote:

> I know they are bad.

They are not totally bad and most real programs wind up
having one or two globals, although usually those globals
are top level container classes, perhaps even an Application
class or similar.

> I am not sure how else to handle this problem.

Others have given lots of ideas but, by way of summary,
there are several common strategies:

1) Use function parameters. Your coding style seems to favour empty 
parameter lists. That's a bad idea. Pass values into functions as 
arguments and return values

def myfunc(n):
     return n+1

foo = 42
foo = myfunc(f00)

rather than

def myfunc():
     global foo
     foo += 1

myfunc()

It avoids hidden, and therefore forgotten, side effects, makes your 
function usable with variables other than foo and is often less code too.

2) Move the  "global" data plus all the functions that access it into a 
class:

class MyFuncs
    def __init__(self, n)
       self.foo = n
    def myfunc(self):
       self.foo += 1

m = MyFuncs(42)
m.myfunc()

Same effect as above but encapsulated in a class so you can create 
multiple instances each with its own value of foo. And you can pass the 
whole class/object around as an argument to other functions (see 
technique 1 above) And you can put it in a module to make the class 
available in other modules.

3) Move all global data into a single module which is then imported by 
any module that needs to access it . This avoids circular dependencies,

By combining these strategies you can minimise the number of globals, 
isolate them in a single module without dependencies  and by making the 
few globals into class instances keep control of the data access.

> One thought was a RAM based SQLite database, but that seems
 > like a lot of work.  I dunno, maybe that is the option.

It is an option, and for things like user ID etc its a valid one. This 
is definitely the best option where the "global" needs to be shared 
across different programs as well as different modules in a single 
program. But it is more work.

-- 
Alan G
Author of the Learn to Program web site
http://www.alan-g.me.uk/