having both dynamic and static variables

[Steven D'Aprano]

On Mon, 07 Mar 2011 13:20:39 -0800, Paul Rubin wrote:

> Steven D'Aprano <steve+comp.lang.python at pearwood.info> writes:
>> but I call that a feature, not a bug. If you want an immutable
>> constant, use a tuple, not a list.
> 
> Nope:
> 
>     L = ([1,2],[3,4])  # tuple
>     L[0].append(5)     # mutate L, in some reasonable sense of "mutate"

You haven't mutated the tuple called "L". You've mutated its internal 
components, which are lists. If you wanted them to also be immutable, you 
should have used tuples :)

It all depends on what sense of mutate you consider "reasonable", but 
once you include any mutable object in a compound object, the whole 
object becomes mutable in *some sense*. If you care about that sense -- 
and you may not -- then you have to ensure that each component that you 
care about immutability is immutable.

The real point I was trying to make is that there are two definitions of 
"constant" that we care about: immutability and resistance to name re-
binding. (Unbindability?) Python already gives us all the tools we need 
for immutability (modulo the usual disclaimers about "we're all adult 
here", "private attributes are private by convention", etc.). What Python 
doesn't have is any way of prohibiting name rebinding short of creating a 
new reserved word like None.



-- 
Steven