[Tutor] advice on global variables

[James Reynolds]

On Tue, Jul 10, 2012 at 3:11 PM, Chris Hare <chare at labr.net> wrote:

>
> I know they are bad.  That is why I would prefer not to use it, but I am
> not sure how else to handle this problem.
>
> In this app, the user must log in.  Once authenticated, they have a userid
> stored in the SQLite database.  Before splitting my app into multiple
> files, I used a global variable.  I know its bad, but it worked.  Now that
> things are split apart, the classes which used it previously now don't see
> it anymore, even using the global keyword.  I think this is the expected
> behavior.  See here
>
> file: a.py
>
> import b
> global_var = "global"
>
> def func1():
>         global global_var
>         print "global var in func1 = %s" % global_var
>
> class intclass:
>         def func2(self):
>                 global global_var
>                 print "global var in intclass = %s" % global_var
>
> print "global_var = %s" % global_var
> func1()
> f = intclass()
> f.func2()
> g = b.extclass()
> g.func3()
>
> file: b.py
>
> class extclass:
>         def func3(self):
>                 global global_var
>                 print "global var in extclass = %s" % global_var
>
> When I run it, I get what I think the expected behavior, that the external
> class ext class won't be able to see the global_var
>
> Big-Mac:t chare$ python a.py
> global_var = global
> global var in func1 = global
> global var in intclass = global
> Traceback (most recent call last):
>   File "a.py", line 18, in <module>
>     g.func3()
>   File "/Users/chare/Development/python/animaltrax/pkg/t/b.py", line 5, in
> func3
>     print "global var in extclass = %s" % global_var
> NameError: global name 'global_var' is not defined
>
> So - my question is this:  how do I solve the problem of multiple classes
> needing to get access to a value which needs to be preserved across the
> lifetime of the running application?
>
> One thought was a RAM based SQLite database, but that seems like a lot of
> work.  I dunno, maybe that is the option.
>
> suggestions, ideas, criticisms are all welcome.  Python code aside, I just
> don't know how to approach this problem in Python.
>
> Thanks, as always for the feedback and guidance.
>
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You should avoid using the global statement.

In your case, I would think you could just add an argument to the method:

class MyObj(object):
    def __init__(self, arg):
        self.arg = arg
    def my_func(self, new_arg):
        self.arg = new_arg

to call it:

arg = 1

m = MyObj(arg)
print m.arg
new_arg = 2
m.my_func(new_arg)
print m.arg
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