A performance issue when using default value

keakon keakon at gmail.com
Mon Feb 1 05:58:50 CET 2010


I've found strange performance issue when using default value, the
test code is list below:

from timeit import Timer

def f(x):
  y = x
  y.append(1)
  return y

def g(x=[]):
  y = []
  y.append(1)
  return y

def h(x=[]):
  y = x
  y.append(1)
  return y

def f2(x):
  y = x
  y.append(1)
  return y + []

def g2(x=[]):
  y = []
  y.append(1)
  return y + []

def h2(x=[]):
  y = x
  y.append(1)
  return y + []

TIMES = 10000
print Timer('f([])','from __main__ import f, g, h').timeit(TIMES)
print Timer('g()','from __main__ import f, g, h').timeit(TIMES)
print Timer('h([])','from __main__ import f, g, h').timeit(TIMES)
print Timer('h()','from __main__ import f, g, h').timeit(TIMES)
print Timer('f2([])','from __main__ import f2, g2, h2').timeit(TIMES)
print Timer('g2()','from __main__ import f2, g2, h2').timeit(TIMES)
print Timer('h2([])','from __main__ import f2, g2, h2').timeit(TIMES)
print Timer('h2()','from __main__ import f2, g2, h2').timeit(TIMES)


I tested it with Python 2.5.4, 2.6.4 and 3.1.1 on Windows XP, and get
almost the same result:
0.00449247041174
0.00439608944712
0.00455867994396
0.00327471787615
0.00791581052899
0.00684919452053
0.00734311204357
0.30974942346

h2() is about 42 times slower than h2([]), but h() is a litter faster
than h([]).

If change TIMES to 20000, other results are 2 times than before, but h2
() is 4 times(about 1.2 sec) than before.

Is there any tricks in it?



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