list_len = len(old_list)
list = [None] * list_len
not
list = [None] * len(old_list)
Use Case:
When I create DataFrames, I’ll create a series of lists to fill. The DataFrame is created by using lists. reviews is a list of HTML blocks. Here’s how I would have normally set up the lists.
texts = [None] * len(reviews)
dates = [None] * len(reviews)
ratings = [None] * len(reviews)
checkin = [None] * len(reviews)
photos = [None] * len(reviews)
A quick timing shows that texts = [None] * len(reviews) is about 22% slower than using a defined variable as the length of the list.
Time to create five empty lists with a length of 20:
- Using review_len: 772 ns ± 5.66 ns
- Using len(reviews): 969 ns ± 13.5 ns
- Using 20: 706 ns ± 14 ns
I’m surprised to see that using a definitive length, 20, didn’t save more time.
Saving nanoseconds may seem like a trivial amount of time, but continually attempting to optimize code is admirable.