Is it faster to use n = len(s) instead of using len(s) directly?

Question

Often to save some time, I would like we to use n = len(s) in my local function. I am curious about which call is faster or they are the same?

while i < len(s):
  # do something

vs

while i < n:
  # do something

There should not be too much difference, but using len(s), we need to reach s first, then call s.length. This is O(1) + O(1). But using n, it is O(1). I assume so.

Answer 1

You're right, here's some benchmarks:

s = np.random.rand(100)
n = 100

Above is setup.

%%timeit
50 < len(s)

86.3 ns ± 2.4 ns per loop (mean ± std. dev. of 7 runs, 10000000 loops each)

Versus:

%%timeit
50 < n

36.8 ns ± 1.15 ns per loop (mean ± std. dev. of 7 runs, 10000000 loops each)

But then again, it's hard to imagine differences on ~60ns level would have affected speed. Unless you're calling len(s) millions of times.