Question

I have recently been experimenting with python generators a bit, and I came across the following curious behaviour, and I am curious to understand why this happens and what is going on:

def generating_test(n): 
    for a in range(n): 
        yield "a squared is %s" % a*a # Notice instead of a**2 we have written a*a

for asquare in generating_test(3): 
    print asquare 

Output:

a squared is 1
a squared is 2a squared is 2

Versus the following script which generates the expected output:

def generating_test(n): 
    for a in range(n): 
        yield "a squared is %s" % a**2 # we use the correct a**2 here

for asquare in generating_test(3): 
    print asquare 

Output:

a squared is 0
a squared is 1
a squared is 4

Answer 1

This doesn’t have anything to do with generators:

>>> a = 2
>>> "a squared is %s" % a
'a squared is 2'
>>> ("a squared is %s" % a)*a
'a squared is 2a squared is 2'
>>> "a squared is %s" % a*a
'a squared is 2a squared is 2'
>>> "a squared is %s" % (a*a)
'a squared is 4'

The % op is performed before the multiplication, using the string and the first a as arguments. Your a**2 works because the ** op with a and 2 as arguments is evaluated before the %.