[Tutor] Creating multiple instance names

[Jeff Shannon]

Lugo Teehalt wrote:

>  Is it possible to use 'for' or 'while' or the like to create
>
> arbitrarilly many named class instances?
>

Yes, you can, but if you need to create a group of class instances, then 
odds are that you're better off keeping them in some sort of collection 
-- either a list or a dictionary.

 >>> class Counter:
...     def __init__(self):
...         self.count = 0
...     def incr(self):
...         self.count += 1
...
 >>> counters = {}
 >>> for i in ['a', 'b', 'c', 'd']:
...     counters[i] = Counter()
...
 >>> counters['a']
<__main__.Counter instance at 0x01798378>
 >>> counters['a'].incr()
 >>> counters['a'].incr()
 >>> counters['c'].incr()
 >>> for name, ctr in counters.items():
...     print "%s - %d" % (name, ctr.count)
...
a - 2
c - 1
b - 0
d - 0
 >>>

While it's possible to futz about with exec and/or globals(), as Bob 
Gailer suggested, I really don't recommend it.  It will lead to code 
that's hard to follow (because it makes it difficult to know what 
is/will be in your namespace), which will lead to bugs.  Those features 
should be reserved for situations where nothing else works, and in this 
situation a collection works splendidly -- I'd even argue that it works 
*better*, by keeping your namespace cleaner and better-organized.  It's 
a good principle to never use black magic when it's not absolutely 
necessary, and I consider both exec and (writing to) globals() to be 
black magic -- or, as I've joked previously, at least very dark grey 
magic. ;)

Jeff Shannon
Technician/Programmer
Credit International