DaveM wrote:
> Although I've programmed for fun - on and off - since the mid 70's, I'm
> definitely an OO (and specifically Python) beginner.
>
> My first question is about global variables. Are they, as I'm starting to
> suspect, a sin against God or just best avoided? Having got my current
> application working using them, I'm not sure whether I want to refactor it,
> but equally, I'd like to write the best code I can.
Use them sparingly IMO
>
> Secondly (and this is related), if you have multiple instances of the same
> class, but need them all to have access to a dictionary created at run-time,
> is there a class way to this without calling the creation method multiple
> times? The best thought I've come up with is for the class instance to check
> its name and only if it's the first one do the dictionary creation, but I
> haven't tried it yet.
>
Make the dictionary an attribute of the class, not the instance. Then it will
be visible by all the instances. See below:
> My third difficulty is with variables in a class. What's the difference
> between the following?:
>
> class Foo:
> i = 12345
> ...
>
this sets i as an attribute of the class.
> class Foo:
> self.i = 12345
> ...
>
this probably raises a NameError, unless you happen to have bound 'self' in the
scope containing Foo, which would be a really bad idea
> class Foo:
> def __init__(self):
> self.i = 12345
> ...
>
this sets i as an attribute of the instance, self, when it is initialized
> class Foo:
> def __init(self):
> i = 12345
> ...
this assigns the name 'i' in the local scope of the __init__ function. It has
no effect on self
So, to get your shared dictionary, you could write:
>>> class A(object):
... shared_dict = {}
...
>>>
>>> a1 = A()
>>> a2 = A()
>>> a1.shared_dict is a2.shared_dict
True
>>> a1.shared_dict["a"] = 42
>>> a2.shared_dict["a"]
42
>>>
Note also, that you create "classic" classes by writing `class Foo:`, and "new
style" classes by writing `class Foo(object):`. There are several subtle
differences between the two, and, since you are just starting, you might as
well
learn with the new ones.
HTH
Michael
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