Overriding a global

[Steven D'Aprano]

On Wed, 14 Dec 2011 13:05:19 +0100, Jean-Michel Pichavant wrote:

> Bad ideas :
> 
> i = 5
> 
> def spam():
>   for i,v in enumerate([1,2,3,4]):
>     for i,v in enumerate(['a','b', 'c']):
>       print i, v
>     print i,v # bad surprise

The bad surprise happens because you are using the same name twice in 
*one* namespace, the local scope. This example has nothing to do with 
local/global name clashes: the existence of global i is irrelevant. 
Python's scoping rules work correctly, and global i is not affected by 
the local i.

Programming languages use multiple namespaces so that you don't need to 
make your variable names globally unique. There are languages that don't 
distinguish between local and global. Python is not one of them. The 
programmer should feel free to use local names without worrying too much 
if they accidentally use a global name.

Having said that, re-using names isn't *entirely* risk free, because if 
you use a global name locally, and then try to *also* access the global 
name, you will fail. This is called shadowing, and the problem with 
shadowing is when you do it by accident. (Newbies are particularly prone 
to this, especially when they call variables "str", "list", etc.) But it 
is the "by accident" part that is dangerous: there is nothing wrong with 
shadowing globals or builtins when you do it by design.


> good ideas :
> 
> # global
> nameThatWillNotBeUsedlocally = 'foo'

Oh please. Names can be too long as well as too short.

 
> def spam():
>   for qtyIndex, quantity in enumerate([5,6,3,1]):
>     for fruitIndex, fruit in enumerate(['orange', 'banana']):
>       print fruitIndex, fruit
>     print qtyIndex, quantity

More sensible naming conventions are to be encouraged, but verbose names 
just for the sake of verbosity is not. spam() is a five line function; if 
the programmer can't keep track of the meaning of loop variables i and j 
over five lines, perhaps they should consider a change of career and get 
a job more suited to their intellectual prowess. I hear McDonalds is 
hiring.

If spam() were larger and more complex, then more expressive names would 
be valuable. But in the simple example you give, it just adds noise.



-- 
Steven