Question

I know how yield works. I know permutation, think it just as a math simplicity.

But what’s yield‘s true force? When should I use it? A simple and good example is better.

Answer 1

yield is best used when you have a function that returns a sequence and you want to iterate over that sequence, but you do not need to have every value in memory at once.

For example, I have a python script that parses a large list of CSV files, and I want to return each line to be processed in another function. I don’t want to store the megabytes of data in memory all at once, so I yield each line in a python data structure. So the function to get lines from the file might look something like:

def get_lines(files):
    for f in files:
        for line in f:
            #preprocess line
            yield line

I can then use the same syntax as with lists to access the output of this function:

for line in get_lines(files):
    #process line

but I save a lot of memory usage.