Valentina Vaneeva wrote:
> Hi,
>
> I'm new to Python and I've found something in its interpreter that I
> don't quite understand, and I don't really know how to correctly
> formulate a search query. Here's the question.
>
> If we have a file module_a.py with the following content:
>
> | #!/usr/bin/env python
> |
> | value = 'initial'
> |
> | def change_value():
> | global value
> | value = 'changed'
>
> and a file module_b.py:
>
> | #!/usr/bin/env python
> |
> | from module_a import value, change_value
> |
> | change_value()
> | print value
>
> Then, if we run module_b.py, it will print "initial". However, if one
> adds to the end of module_a.py this line:
>
> | change_value()
>
> The script module_b.py will print "changed".
>
> It seems that in the first case change_value() called in module_b.py
> ignores the global statement. Is it so? Why? What happens in the second
> case? I really don't get it.
>
In Python, there are not any truly "global" variables as you're thinking
of them. Instead, each module has it's own global space. You are
tripping over the fact that module_a and module_b have their own
separate global spaces.
The variable "value" is global in module_a, and "change_value" will
always refer to that variable.
However, in module_b, when you
from module_a import value, change_value
you have created two new variables global to module_b that references
values from the other module.
Now running change_value (which changes "value" in module_a has no
affect on the version of "value" in module_b. Your print is then
printing value from module_b -- the unchanged version.
If you truly want to examine value in module_a, the you have to access it as
import module_a
module_a.change_value()
print module_a.value
You can do both
import module_a
from module_a import change_value
but that's probably confusing and generally considered a bad idea.
Ideas:
1 If you want module_a to manage such a global variable, you could
include functions for getting, and setting it, as well as your function
for changing it's value.
2. You could also dispense with use of global values -- various OOP
techniques could help there.
Gary Herron
> Thanks!
>
> Cheers,
> Valia
>
>
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