17.3 Evaluating Predicate Tests Across Sequences
Credit: Jon Dyte
17.3.1 Problem
You need to
know whether all items in a sequence satisfy a certain predicate or
if only some of them do.
17.3.2 Solution
The simplest approach for either problem is to loop on the sequence
and return a result as soon as it's known, just as
the and and or Python operators
shortcircuit logical evaluation:
def every (pred, seq):
""" true if pred(x) is true for all x in seq, else false """
for x in seq:
if not pred(x): return 0
return 1
def any (pred, seq):
""" false if pred(x) is false for all x in seq, else true """
for x in seq:
if pred(x): return 1
return 0
17.3.3 Discussion
Often, it is useful to know whether all elements of a sequence meet
certain criteria or if only some do. The two functions
every
and any do just that, with the simplest, and thus
clearest, approach:
>>> every(lambda c: c > 5, (6, 7, 8, 9))
1
>>> every(lambda c: c > 5, (6, 4, 8, 9))
0
>>> any(lambda c: c > 5, (6, 7, 8, 9))
1
>>> any(lambda c: c < 5, (6, 7, 8, 9))
0
If you want to get fancy, here are two more techniques (perhaps with
some performance gain, though you shouldn't take my
word for it梐lways measure performance for the specific cases
you need):
def every(pred, seq): return len(seq) == len(filter(pred, seq))
def any(pred, seq): return len(filter(pred, seq))
or:
import operator
def every(pred, seq):
return reduce(operator.and_, map(pred, seq))
def any(pred, seq):
return reduce(operator.or_, map(pred, seq))
Functional forms are elegant and often fast, as long as they do not
involve a lambda.
