Best practice: Sharing object between different objects

[Mark Lawrence]

On 23/02/2015 20:02, sohcahtoa82 at gmail.com wrote:
> On Monday, February 23, 2015 at 10:45:59 AM UTC-8, Michael Torrie wrote:
>> On 02/23/2015 11:10 AM, Rob Gaddi wrote:
>>> So I'd solve it with module level global variables.  It's semi-frowned
>>> upon on software stuff because it creates an unintentional shared state
>>> between different modules, but you really HAVE a shared state, so it
>>> needs to be dealt with.
>>
>> I would also do it with a module attribute.  In my mind that's exactly
>> the right way to go.
>>
>> But I disagree that it's frowned on or a bad thing. A module is a
>> completely appropriate place to store state.  In fact a module is an
>> object, but a special one that can only be instantiated once.  So as far
>> as patterns go, a module is a singleton. Almost any time in Python you
>> have something that you want to have exactly one instance of in your
>> program, you don't want to define a class but rather just use a module.
>>
>> Any code in a module can be considered the constructor. It executes only
>> once in your program when the module is first imported, no matter how
>> many times its imported after that.
>>
>> I often use a module to store configuration that is shared across
>> modules in my projects.
>
> That behavior always trips me up.  My intuition tells me that every time you import a module, it re-runs the code in the module.  So if I had a simple module named myModule.py that had a single line with `myInt = 1`, then I would *EXPECT* this behavior:
>
>>>> import myModule
>>>> myModule.myInt
> 1
>>>> myModule.myInt = 2
>>>> myModule.myInt
> 2
>>>> import myModule
>>>> myModule.myInt
> 1
>
> It seems slightly counter-intuitive, but the *ACTUAL* behavior (where the final line prints 2) does allow for something nice: Easy use of global variables.
>
> What's REALLY interesting is that this happens:
>
>>>> import myModule
>>>> myModule.myInt
> 1
>>>> myModule.myInt = 2
>>>> myModule.myInt
> 2
>>>> del myModule
>>>> import myModule
>>>> myModule.myInt
> 2
>
> I would REALLY expect that deleting the module object and then re-importing would reset that variable.  And then...
>
>>>> del myModule.myInt
>>>> del myModule
>>>> import myModule
>>>> myModule.myInt
> Traceback (most recent call last):
>    File "<stdin>", line 1, in <module>
> AttributeError: 'module' object has no attribute 'myInt'
>
> So yeah...modules are really only loaded once unless you use reload/imp.reload/importlib.reload.
>
> But yeah, it seems slightly counter-intuitive to me at first, but certainly has its uses.
>

Rule 1) Don't expect anything of Python, always read the docs.
Rule 2) If in doubt always refer to rule 1)

-- 
My fellow Pythonistas, ask not what our language can do for you, ask
what you can do for our language.

Mark Lawrence