How come this sum() syntax right in Python?-CodePudding

I found this simple code snippet from somewhere, but I don't understand how come this sum() syntax works.

total = sum(1 for _ in [1,2,3,4,5])
print(total)      # 5

for _ in [1,2,3,4,5] is nothing but the looping five times.

So the code snippet loops five times and add 1 for each loop so becomes 5 I guess.

I'm not sure about while looping five times in for _ in [1,2,3,4,5] what's happening then with 1?

According to the syntax of sum(iterable, start), the first argument should be iterable, but 1 is int. How come this works based on the sum syntax. How this code internally works? I'm confused.

CodePudding user response：

1 for _ in [1,2,3,4,5] is an iterator which is similar to

def my_gen():
    for _ in range(5):
        yield 1

This returns 1 five times. So the line can be written as

sum((1,1,1,1,1))

CodePudding user response：

Does it help if you break it down:

(1 for _ in [1,2,3])

or even

[1 for _ in [1,2,3]]

works as counting the elements of the list. So providing the iterable the result of that expression ([1,1,1]) to the sum() is providing the expected result.