[Noisebridge-discuss] PyClass 2011-04-18

[Seth David Schoen]

Timmy Shih Jun Yee writes:

> * Short-circuit evaluation (i.e. Lazy evaluation)

I don't agree that short-circuit and lazy evaluation are the same thing.

https://secure.wikimedia.org/wikipedia/en/wiki/Short-circuit_evaluation
https://secure.wikimedia.org/wikipedia/en/wiki/Lazy_evaluation

Short-circuit evaluation is what causes

def cat():
    print "meow"
    return True

def dog():
    print "woof"
    return True

some_animal = cat() or dog()

to print only "meow" and not also "woof" (because the interpreter
knows that the expression will be true without needing to look at
the return value of dog()).

Lazy evaluation is what allows something like

from itertools import takewhile, ifilter, count

def even(x):
    return x % 2 == 0

def less_than_ten(x):
    return x < 10

print list(takewhile(less_than_ten, ifilter(even, count())))

to work, because it doesn't get any more values out of count() than
are necessary for the caller's purposes, even though count() is capable
of providing an unlimited number of values.

-- 
Seth David Schoen <schoen at loyalty.org> | Qué empresa fácil no pensar en
     http://www.loyalty.org/~schoen/   | un tigre, reflexioné.
     http://vitanuova.loyalty.org/     |            -- Borges, El Zahir