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Date:	Wed, 13 Apr 2005 22:28:48 PDT7
From:	Sheridan George <s-geo@usa.net >
Reply-To:	scoug-programming@scoug.com
To:	scoug-programming@scoug.com
Subject:	SCOUG-Programming: Progress--Step 3

Content Type: text/plain

Lynn H. Maxson wrote:
> Peter,
>

>
> We have had two very excellent presentations by Greg Smith
> and Sheridan George on PERL. PERL has two forms of a list

Err...this should be Python. PERL is what you throw before swine.

> aggregate, one variable form and one constant form. Each
> looks the same except for boundary notation: (....) variable
> and [....] constant (if I have that right). Now I find the idea of

I need to tweek your understanding a little. Python tuples ( boundary notation (.....) ) are not
constant. They are immutable though it says in fine print. However, Python does have methods to
change a tuple. Under the covers it creates a new tuple and garbage collects the former one.

For example:
>>> x=(123,"abc") #create a tuple
>>> print type(x)

>>> x=x+("gfd",9876) #add a tuple to an immutable tuple
>>> print x
(123, 'abc', 'gfd', 9876) #worked OK
>>> print x[2] #pick one element of the tuple - the Python term is slice
gfd
>>> x[2]=('zxc',768) #try to change that element
Traceback (most recent call last):
File "", line 1, in ?
TypeError: object doesn't support item assignment

So, we see tuples are changeable (by method but not assignment) even though they are said to be
immutable.

Now for lists ( boundary notation [.....] ):
>>> a=[123, "abc"] #create a list
>>> print type(a)

>>> a=a+['tre',987] #add to list
>>> print a
[123, 'abc', 'tre', 987]
>>> print a[2] #pick an element
tre
>>> a[2]=921 #change an element by assignment
>>> print a
[123, 'abc', 921, 987] #works

Python lists are considered mutable.

The immutable constructs are required internally because they can be safely hashed. The Python
dictionary (a {key:value} pair) uses a hash for its key part. Dictionaries are the underpinnings of
Python. They are used for stacks and Name Space. Every time a class or function is defined a new
dictionary is created. That dictionary holds all of the internals for that class or function. And
as I understand it when a class is instantiated yet another dictionary is created.

>
> In short I don't believe in a list constant or the need for it. If
> you want something constant, you don't change it. So it

This is how Python works. Python does not have a constant construct. If you want a constant simply
don't change it. The convention is to all caps the var name to indicate it is to be considered a
constant. e.g. CONSTANT = 12345

Sheridan

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