# Python summing in loop vs sum() performance

**URL:** <https://discuss.python.org/t/python-summing-in-loop-vs-sum-performance/25440>\
**Category:** Python Help\
**Created:** [April 3, 2023, 7:59pm UTC](https://discuss.python.org/t/python-summing-in-loop-vs-sum-performance/25440 "2023-04-03T19:59:17Z")\
**Posts on this page:** 4\
**Page:** 1

<div class="post-metadata">

**Author:** ![DamirHanov](https://sea2.discourse-cdn.com/flex002/user_avatar/discuss.python.org/damirhanov/32/12200_2.png) [@DamirHanov](https://discuss.python.org/u/DamirHanov)\
**Post date:** [April 3, 2023, 7:59pm UTC](https://discuss.python.org/t/python-summing-in-loop-vs-sum-performance/25440/1 "2023-04-03T19:59:17Z")

</div>

Hey everyone!

Recently I was playing with python. And noticed in my opinion strange behaviour.

I have 4 functions:

```python
def t1():
    s = time.perf_counter_ns()
    counter = 0
    for _ in range(10_000_000):
        counter += 1
    e = time.perf_counter_ns()
    print(counter)
    return int((e - s) / 1_000_000)

def t2():
    s = time.perf_counter_ns()
    counter = sum(1 for _ in range(10_000_000)
    e = time.perf_counter_ns()
    print(counter)
    return int((e - s) / 1_000_000)

def t11():
    s = time.perf_counter_ns()
    counter = 0
    for i in range(10_000_000):
        counter += i
    e = time.perf_counter_ns()
    print(counter)
    return int((e - s) / 1_000_000)

def t22():
    s = time.perf_counter_ns()
    counter = sum(range(10_000_000))
    e = time.perf_counter_ns()
    print(counter)
    return int((e - s) / 1_000_000)

```

When I run t11 and t22 I get that time t11 \> t22. But when I run t1 and t2 I get the opposite.

I tried to run it on different machines - Intel and AMD, both are Win11 with Python 3.11.0 and get the same result in both cases. Below is result for AMD run.  
 ![image](https://us1.discourse-cdn.com/flex002/uploads/python1/original/2X/9/96365fad74872ed4cb4bfa6cc6ba829330ffd2c9.png)

So, I just wonder why this is happening.

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<div class="post-metadata">

**Author:** ![tjreedy](https://sea2.discourse-cdn.com/flex002/user_avatar/discuss.python.org/tjreedy/32/137_2.png) [@tjreedy](https://discuss.python.org/u/tjreedy)\
**Post date:** [April 3, 2023, 8:22pm UTC](https://discuss.python.org/t/python-summing-in-loop-vs-sum-performance/25440/2 "2023-04-03T20:22:01Z")

</div>

The comparisons are computationally not the same. Functions t1 and t11 directly iterate the range. In t22, sum directly iterates the range. In t2, sum iterates a generator that iterates the range, so there in effect twice as many next calls, which is slower. Change t2 to directly iterate 1 iterator, as t1 does, such as

```auto
def t2b():
    onelist = [1] * 10_000_000
    s = ...
    counter = sum(onelist)
    ....

```

and I strongly suspect that it will beat t1.

EDIT: spelling above, plus add … Or slow down t1 by doing the same double iteration:

```auto
def t1b():
    s = ...
    counter = 0
    for _ in (1 for i in range(10_000_000))
    ...

```

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<div class="post-metadata">

**Author:** ![DamirHanov](https://sea2.discourse-cdn.com/flex002/user_avatar/discuss.python.org/damirhanov/32/12200_2.png) [@DamirHanov](https://discuss.python.org/u/DamirHanov)\
**Post date:** [April 3, 2023, 8:40pm UTC](https://discuss.python.org/t/python-summing-in-loop-vs-sum-performance/25440/3 "2023-04-03T20:40:21Z")

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Thank you for reply! I understand that summing a list as you proposed is faster, but it consumes more memory and I tried to avoid that.

I thought that compiler could optimise double generator to something similar to t22 😅

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<div class="post-metadata">

**Author:** ![tjreedy](https://sea2.discourse-cdn.com/flex002/user_avatar/discuss.python.org/tjreedy/32/137_2.png) [@tjreedy](https://discuss.python.org/u/tjreedy)\
**Post date:** [April 3, 2023, 8:43pm UTC](https://discuss.python.org/t/python-summing-in-loop-vs-sum-performance/25440/4 "2023-04-03T20:43:21Z")

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See edit for a way to do a fair comparison without the list.