Why is subtraction faster than addition in Python?

Question

I was optimising some Python code, and tried the following experiment:

import time

start = time.clock()
x = 0
for i in range(10000000):
    x += 1
end = tim         


        

Answer 1

That would be remarkable, so I have thoroughly evaluated your code and also setup the expiriment as I would find it more correct (all declarations and function calls outside the loop). Both versions I have run five times.

• Running your code validated your claims: -= takes constantly less time; 3.6% on average
• Running my code, though, contradicts the outcome of your experiment: += takes on average (not always) 0.5% less time.

To show all results I have put plots online:

• Your evaluation: http://bayimg.com/kadAeaAcN
• My evaluation: http://bayimg.com/KadaAaAcN

So, I conclude that your experiment has a bias, and it is significant.

Finally here is my code:

import time

addtimes = [0.] * 100
subtracttimes = [0.] * 100

range100 = range(100)
range10000000 = range(10000000)

j = 0
i = 0
x = 0
start = 0.


for j in range100:
 start = time.clock()
 x = 0
 for i in range10000000:
  x += 1
 addtimes[j] = time.clock() - start

for j in range100:
 start = time.clock()
 x = 0
 for i in range10000000:
  x -= -1
 subtracttimes[j] = time.clock() - start

print '+=', sum(addtimes)
print '-=', sum(subtracttimes)