How to reassign the value of the variable on runtime?

[Ivan Evstegneev]

-----Original Message-----
From: Python-list
[mailto:python-list-bounces+webmailgroups=gmail.com at python.org] On Behalf Of
Chris Angelico
Sent: Saturday, August 29, 2015 19:02
Cc: python-list at python.org
Subject: Re: How to reassign the value of the variable on runtime?

On Sun, Aug 30, 2015 at 1:46 AM, Ivan Evstegneev <webmailgroups at gmail.com>
wrote:
> It looks like, when the module is loaded (imported) for the first 
> time, all the functions that are defined within it and using a  global 
> varialbles as their defaults, they would keep the first value of this
globals.

That's correct. When you define a function, it snapshots the default values
for its arguments; if you say "engine=engine_object", Python will evaluate
the name "engine_object" at the time when the 'def'
statement is executed (which in this case is module load) and save that as
the default value for the argument "engine". Changing the value of
engine_object won't change that.

> So assumed you've already knew about this behavior and suggested to 
> use "lazy links". Am I right?

Right. If you use "engine=None", then Python will evaluate the expression
"None", which (what a surprise!) means the special object None. There's only
one such object, and you'll never be replacing it.
Then, whenever the function is called, you say "hey, if this thing's None,
go and grab the one from the global"; and since that happens when the
function's _called_ rather than when it's _defined_, it uses the value of
engine_object at that time.

> Still what is the difference between the way proposed:
>
> ***************************************************
>>def current_data_export(engine=None, meta=None):
>>    if engine is None: engine = engine_object
>>   if meta is None: meta = meta_object
> ****************************************************
>
> and the direct use of these globals within the function?
> Like so:
>
> ******************************************************
> def current_data_export():
>         engine_object.some_method_used
>         meta_object.some_metheod_used
> ******************************************************
>
>  In both cases the engine and meta objects will be searched in 
> outerscope, what is the difference then?

Correct, both will be fetched from the outer scope. The difference is that
you can override them using the first form, by calling the function with
arguments:

current_data_export(some_other_engine, some_other_meta)

which you can't do if you use the globals directly. But both forms do use
the current values of the globals as at when the function is called.

ChrisA
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Thanks a lot (really).


As for the globals?  Each function that need to modify the globals should
contain a "global bar" inside its body? 

Ivan.