Question

I’m reading about the yield keyword in python, and trying to understand running this sample:

def countfrom(n):
    while True:
        print "before yield"
        yield n
        n += 1
        print "after yield"

for i in countfrom(10):
    print "enter for loop"
    if i <= 20:
        print i
    else:
        break

The output is:

before yield
enter for loop
10
after yield
before yield
enter for loop
11
after yield
before yield
enter for loop
12
after yield
before yield
enter for loop
13
after yield
before yield
enter for loop
14
after yield
before yield
enter for loop
15
after yield
before yield
enter for loop
16
after yield
before yield
enter for loop
17
after yield
before yield
enter for loop
18
after yield
before yield
enter for loop
19
after yield
before yield
enter for loop
20
after yield
before yield
enter for loop

It looks like the yield will return the specified value, and will continue runnning the function till the end (in a parallel thread, maybe). Is my understand correct?

If you could answer this without mentioning “generators”, I would be thankful, because I’m trying to understand one at a time.

Answer 1

You can think of it as if the function which yields simply “pauses” when it comes across the yield. The next time you call it, it will resume after the yield keeping the state that it was in when it left.