maybe_raise.py

from random import random

def maybe_raise(chance, n):
	for x in xrange(n):
		try:
			if random() < chance:
				raise ValueError("derp")
		except ValueError:
			pass

from timeit import timeit

number = 5000

for c in [1, 0.9, 0.3, 0.1, 0]:
	print "%d%% raise exceptions: time %.2f sec" % (c * 100, timeit(lambda:maybe_raise(c, 1000), number=number))

results.txt

100% raise exceptions: time 5.79 sec
90% raise exceptions: time 4.01 sec
30% raise exceptions: time 1.71 sec
10% raise exceptions: time 0.91 sec
0% raise exceptions: time 0.58 sec