[Tutor] Creating a complex python object

[Bob Gailer]

At 12:13 PM 8/13/2004, Jeff Shannon wrote:
>Chad Crabtree wrote:
>>  >In the mean time I'll keep lurking on this list, there's still a
>>lot to python that baffles me (like just what is the difference between a
>>tuple and a list?  :-)   ).
>>I thought I would answer the tuple list thing.  I did not understand
>>why have tuple's until I learned that tuples being immutable are faster
>>and easier on memory.
>
>Actually, speed and memory considerations are only incidental.
>
>One of the most important reasons that Python has both (mutable) lists and 
>(immutable) tuples is because dictionaries don't work with mutable 
>keys.  If you stuck something into a dictionary that was keyed on a list, 
>and then that list changed, you'd never be able to get that dictionary 
>value back.  But dictionaries are an important part of Python (many 
>internal data structures are effectively dictionaries, among other 
>things), and it's very valuable to be able to use a group  of items as 
>keys -- for example, a dictionary showing grid locations is much cleaner 
>if you can key off of (x, y), rather than having a dictionary at x that 
>has another dictionary that's keyed off of y.
>
>So, dictionaries are the reason that we have tuples.  But once we *have* 
>them, they tend to grow other differences...
>
>The common usage, now, is that lists usually contain a sequence of similar 
>things, whereas tuples are usually more like a record, grouping together 
>dissimilar things that are conceptually related. Thus, for example, in the 
>date-time tuple returned by time.gmtime() each position within the tuple 
>represents a different thing.  In contrast, each position in a list 
>represents a different instance, but all of the contents are the same type 
>of thing.  This distinction is just convention, of course, and there's 
>nothing forcing you to follow it, but you'll find that just about every 
>standard library module (and most other code, as well) will follow this 
>guideline.

Darn - and just today I created a mechanism using a list in which the 1st 
element is a string and the rest integer. Sigh.

Bob Gailer
bgailer at alum.rpi.edu
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