timeit

Sometimes speed is important. The timeit modules lets you find how fast you are.

from timeit import Timer

def fast_fib(n):
  if n < 2:
      return 1

  a, b = 1, 1
  for i in range(n - 1):
      a, b = b, a + b

  return b

def slow_fib(n):
  if n < 2:
      return 1

  return slow_fib(n - 1) + slow_fib(n - 2)


INDEX = 20
TIMES = 100

fast_timer = Timer("fast_fib(INDEX)", "from __main__ import fast_fib, INDEX")
slow_timer = Timer("slow_fib(INDEX)", "from __main__ import slow_fib, INDEX")

print "slow:", slow_timer.timeit(TIMES) / TIMES
print "fast:", fast_timer.timeit(TIMES) / TIMES

On my machine this gives:

fast: 5.11884689331e-06
slow: 0.00996325016022

However sometimes you to send your function some more complex data, plus you don't want to add too much timing code into the top level of your module.
One way to do it is:

def benchmark():
  benchmark.index = 20
  timer = Timer("fast_fib(benchmark.index)",
                "from __main__ import fast_fib, benchmark")
  num_runs = 100

  print timer.timeit(num_runs) / num_runs

def main(argv=None):
  if argv is None:
      import sys
      argv = sys.argv

  from optparse import OptionParser

  parser = OptionParser("usage: %prog [options] MESSAGE")
  parser.add_option("--benchmark", help="run benchmark",
          dest="benchmark", action="store_true", default=0)

  opts, args = parser.parse_args(argv[1:])

  if opts.benchmark:
      benchmark()
      raise SystemExit()

  if len(args) != 1:
      parser.error("wrong number of arguments") # Will exit

  # Do main program stuff here
  try:
      print fast_fib(int(args[0]))
  except ValueError:
      raise SystemExit("error: %s - bad number" % args[0])

if __name__ == "__main__":
  main()