A dumb question about a class
Steven Bethard
steven.bethard at gmail.com
Mon Aug 13 00:09:52 CEST 2007
Dick Moores wrote:
> I'm still trying to understand classes. I've made some progress, I
> think, but I don't understand how to use this one. How do I call it, or
> any of its functions? It's from the Cookbook, at
> <http://aspn.activestate.com/ASPN/Cookbook/Python/Recipe/523048>.
The short answer is that use should look something like::
>>> plist = PrimeList()
>>> plist.contains(32)
False
>>> plist.contains(23)
True
But this doesn't seem like a particularly good recipe. Seems like you
would really rather be writing code like::
>>> plist = PrimeList()
>>> 1 in plist
False
>>> 2 in plist
True
>>> 22 in plist
False
>>> 23 in plist
True
>>> 782 in plist
False
>>> 787 in plist
True
Here's how I'd write the recipe::
import itertools
def iter_primes():
# an iterator of all numbers between 2 and +infinity
numbers = itertools.count(2)
# generate primes forever
while True:
# get the first number from the iterator (always a prime)
prime = numbers.next()
yield prime
# remove all numbers from the (infinite) iterator that are
# divisible by the prime we just generated
numbers = itertools.ifilter(prime.__rmod__, numbers)
class PrimeList(object):
def __init__(self):
# infinite iterator of primes
self._prime_iter = iter_primes()
# the last prime we've seen
self._last_prime = None
# all primes seen so far
self._prime_set = set()
# add the first prime (so that _last_prime is set)
self._add_prime()
def __contains__(self, n):
# add primes to the list until we exceed n
while n > self._last_prime:
self._add_prime()
# return True if n is one of our primes
return n in self._prime_set
def _add_prime(self):
# take a prime off the iterator and update the prime set
self._last_prime = self._prime_iter.next()
self._prime_set.add(self._last_prime)
STeVe
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