Performance of string concatenation

string_concat_performance.py

import timeit

def concat1(N):
  res = 'a'
  for _ in [0] * N:
    res += 'a'

def concat2(N):
  res = 'a'
  for _ in [0] * N:
    unused = res
    res += 'a'

nums_iter = [1000, 2000, 5000, 10000, 20000, 50000, 100000]

print("concat1")
for N in nums_iter:
  exec_time = min(timeit.repeat('concat1({0})'.format(N), globals=globals(), number=1, repeat=5))
  print("N = {0}: exec_time = {1:.6f}".format(N, exec_time))

print()

print("concat2")
for N in nums_iter:
  exec_time = min(timeit.repeat('concat2({0})'.format(N), globals=globals(), number=1, repeat=5))
  print("N = {0}: exec_time = {1:.6f}".format(N, exec_time))

# concat1
# N = 1000: exec_time = 0.000091
# N = 2000: exec_time = 0.000190
# N = 5000: exec_time = 0.000468
# N = 10000: exec_time = 0.000915
# N = 20000: exec_time = 0.001706
# N = 50000: exec_time = 0.004765
# N = 100000: exec_time = 0.010892

# concat2
# N = 1000: exec_time = 0.000138
# N = 2000: exec_time = 0.000278
# N = 5000: exec_time = 0.000770
# N = 10000: exec_time = 0.001836
# N = 20000: exec_time = 0.006016
# N = 50000: exec_time = 0.043246
# N = 100000: exec_time = 0.165473