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.
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.