Python: why do two seemingly the same codes work different times? [duplicate]

Question

I was conducting tests on Python (3.8.6) work speed. The test case was matrix per vector multiplication with sizes 10.000x10.000 and 10.000 correspondingly. The matrix and the vector were randomly filled with float numbers.

Firstly, I tried this code:

import time
import numpy as np

def str_per_vec(a, b, n):
    res = 0
    for i in range(n):
        res += a[i] * b[i]
        
    return res

N = 10000

A = np.random.randn(N, N)
b = np.random.randn(N)
correct_answer = A @ b

A = A.tolist()
b = b.tolist()
c = [None] * N

start = time.perf_counter()

for i in range(N):
    c[i] = str_per_vec(A[i], b, N)

end = time.perf_counter()    
    
assert np.allclose(c, correct_answer)
print("Time:", end - start)

And the output was "Time: 6.585052800000001"

Then I tried another code. In fact, I just removed the function and wrote it in the loop itself:

import time
import numpy as np

N = 10000

A = np.random.randn(N, N)
b = np.random.randn(N)
correct_answer = A @ b

A = A.tolist()
b = b.tolist()
c = [None] * N

start = time.perf_counter()

for i in range(N):
    buf = 0
    a = A[i]
    for j in range(N):
        buf += a[j] * b[j]
    c[i] = buf

end = time.perf_counter()    
    
assert np.allclose(c, correct_answer)
print("Time:", end - start)

And this time the output was "Time: 12.4580008".

So, I just moved code from the function to the loop, but it took twice more time to execute! I'm really confused with it, because I have no idea why this happened. If someone proficient in Python could help me, I would be very grateful!

P.S. I conducted tests several times and the results were stable.

Answer 1

Thanks to Anthony Sottile and Kelly Bundy for their responses in the comments, they have clarified everything. The thing was in local variables and global variables (see Performance with global variables vs local). The first code use local variables inside a function, whereas the second one uses global variables. And since access to local variables is in principle faster, than to global variables, the first code worked faster. If the second code is wrapped into a function, it works the same time the first does.