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Date: Mon, 11 Nov 2013 21:39:27 +1100
Subject: Re: New user's initial thoughts / criticisms of Python
From: Chris Angelico <rosuav@gmail.com>
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On Mon, Nov 11, 2013 at 9:09 PM,  <lorenzo.gatti@gmail.com> wrote:
> Regarding the "select" statement, I think the most "Pythonic" approach is using dictionaries rather than nested ifs.
> Supposing we want to decode abbreviated day names ("mon") to full names ("Monday"):

That's an obvious mapping, though. If you're using a select/switch
statement to handle straight-forward one-to-one mappings, then yes,
obviously the better way to do it is to use a dictionary. In the more
general sense, a switch/case block is much more directly translated
into if/elif/else statements. You can't, for instance, build up a
dictionary that handles inequalities, but you can do that with elif.

That is, normally you can't. I have occasionally built up mappings
that handle inequalities - it's a form of denormalization. Consider
the following logic:

A 'minor weapon' is based on a roll of a 100-sided dice. If it's 01 to
70, "+1 weapon: 2,000gp [weapon]"; if it's 71 to 85, "+2 weapon:
8,000gp [weapon]"; if 86 to 90, "Specific weapon [minor specific
weapon]"; and if 91 to 100, "Special ability [minor special weapon]
and roll again".

My code to handle that starts out with this array:

"minor weapon":({
    70,"+1 weapon: 2,000gp [weapon]",
    85,"+2 weapon: 8,000gp [weapon]",
    90,"Specific weapon [minor specific weapon]",
    100,"Special ability [minor special weapon] and roll again",
}),

(that's Pike; in Python it'd be a list, or maybe a tuple of tuples),
and denormalizes it into a lookup table by creating 70 entries quoting
the first string, 15 quoting the second, 5, and 10, respectively. So,
with a bit of preprocessing, a lookup table (which in this case is an
array (list), but could just as easily be a dict) can be used to
handle inequalities. But this is because lookup tables can be treated
as data, where if/elif/else blocks have to be code; there are roughly
42 million such lookup tables in the code I snagged that from, and
having code for each one would work out far less manageable. Normally,
you'll want to render inequalities with code as if/elif.

ChrisA