Newcomer question wrt variable scope/namespaces

[Gary Duzan]

Florian Daniel Otel wrote:
> Gary,
> 
> First of all, many  thanks for the reply. Do I understand it correctly
> that actually the rule has to be refined as pertaining  to the (so
> called) "immutable" types (like e.g.  integers, tuples/strings)
> whereas lists and dictionaries are "mutable" types and the said
> scoping rule does not apply ?

    The rule has to do with modifying namespaces. Setting a variable to 
a value modifies the variable's binding in its namespace; it doesn't 
matter whether the variable references a mutable or immutable type:

=========================================================================
$ python
Python 2.4.1 (#1, May 27 2005, 18:02:40)
[GCC 3.3.3 (cygwin special)] on cygwin
Type "help", "copyright", "credits" or "license" for more information.
 >>> a = 1
 >>> b = [1,2,3]
 >>> def foo():
...     a = 9
...
 >>> def bar():
...     b = [7,8,9]
...
 >>> foo()
 >>> print a
1
 >>> bar()
 >>> b
[1, 2, 3]
 >>>
=========================================================================

Note that there is no error, but the binding affect each function's 
local namespace instead of the global one. However, modifying a mutable 
type, in this case a list, is not a namespace binding operation, i.e., 
it doesn't make the variable point to a different object. Since you only 
need to access a variable's contents to modify the object to which it 
refers, you can do this:

=========================================================================
 >>> def blah():
...     b[2] = 6
...
 >>> blah()
 >>> b
[1, 2, 6]
 >>>
=========================================================================

It is easy to think that the scoping rules give you read-only behavior, 
but with mutable objects it just isn't correct.

					Gary Duzan
					Motorola CHS