Why is Python's built-in sum much slower than manual summation?

Question

Here is my simple code example:

import time

t0 = time.time()
s = 0
for i in range(1000000):
    s += i
t1 = time.time()

print(s, t1 - t0)

t0 = time.time()
s = sum(i for i in range(1000000))
t1 = time.time()

print(s, t1 - t0)

On my computer (with Python 3.8) it prints:

499999500000 0.22901296615600586
499999500000 1.6930372714996338

So, doing += a million times is 7 times faster than calling sum? That is really unexpected. What is it doing?

---

Edit: I foolishly allowed a debugger to attach to the process and interfere with my measurements, which was in the end the cause of the slowness. With the debugger out, the measurements are no longer so unpredictable. As some of the answers are clearly showing, what I observed shuold not happen.

Answer 1

Let's use timeit for proper benchmarking and to make it easy to also compare different Python versions, let's run this in Docker containers:

so62514160.py

N = 1000000

def m1():
    s = 0
    for i in range(N):
        s += i

def m2():
    s = sum(i for i in range(N))

def m3():
    s = sum(range(N))

so62514160bench.sh

for image in python:2.7 python:3.6 python:3.7 python:3.8; do
    for fun in m1 m2 m3; do
        echo -n "$image" "$fun "
        docker run --rm -it -v $(pwd):/app -w /app -e PYTHONDONTWRITEBYTECODE=1 "$image" python -m timeit -s 'import so62514160 as s' "s.$fun()"
    done
done

results on my machine:

python:2.7 m1 10 loops, best of 3:  43.5 msec per loop
python:2.7 m2 10 loops, best of 3:  39.6 msec per loop
python:2.7 m3 100 loops, best of 3: 17.1 msec per loop
python:3.6 m1 10 loops, best of 3:  41.9 msec per loop
python:3.6 m2 10 loops, best of 3:  46 msec per loop
python:3.6 m3 100 loops, best of 3: 17.7 msec per loop
python:3.7 m1 5 loops, best of 5:   45 msec per loop
python:3.7 m2 5 loops, best of 5:   40.7 msec per loop
python:3.7 m3 20 loops, best of 5:  17.3 msec per loop
python:3.8 m1 5 loops, best of 5:   48.2 msec per loop
python:3.8 m2 5 loops, best of 5:   44.6 msec per loop
python:3.8 m3 10 loops, best of 5:  19.2 msec per loop

plot