Why does nobody mention how slow Python loops actually are?

I was working on a project last night that had to iterate through 10,000 items. Took like 15 seconds to finish - I thought I had a bug or something. Did I miss the memo on optimizing loops or is this just normal for beginners?

3 comments

3 Comments

claire_gibson25d ago

The built-in range() function can actually be a hidden bottleneck if you're not careful. Something like using a while loop with a counter variable might feel more manual but can sometimes run faster in specific cases since you control the increment step directly. Another thing that tripped me up early on was forgetting to move function calls outside the loop. If your loop calls len(some_list) every single iteration instead of storing it in a variable first, that adds a ton of overhead. Take this with a grain of salt though because it really depends on what you're doing inside the loop.

theagibson25d ago

Range() being a bottleneck is the kind of thing I'd only figure out after my code took forever to run... and then I'd slap my forehead because I totally forgot to hoist my len() call out of the loop again. I've literally sat there timing while loops vs for loops for simple counter stuff and felt like a real goof when the difference was like 0.2 seconds for a million iterations. But hey, at least my debugging sessions have taught me to always stash that list length in a variable before the loop starts... or just code myself into a corner and remember the hard way.

miles_roberts24d ago

Funny enough I had this exact realization last week while debugging a loop that was supposed to process about 50,000 records. I had range() in there with a step of 1, but I was also checking if i % 2 == 0 inside the loop to skip odd indexes. My buddy watched me for ten minutes then just said "why not use range(0, len(list), 2)?" and I felt like such an idiot. The while loop version I tried after that was maybe 0.05 seconds faster but the code looked like a mess with manual incrementing and a break condition. Honestly I spent more time arguing with myself over which was "better" than the actual time savings were worth.