Please explain uncommon way of declaring and using variable [function.variable]

[esphi]

I came across similar code as bellow. It declare a variable foo.y and foo.z. They can be called outside the function.
I don't see this being explained in any tutorial.

Is this a common practice?
Can anyone please explain or give any lead on manual or info about this, thank you.

def foo(x):
    foo.y = 2
    foo.z = foo.y ** x 
    return x * foo.y

print(f'foo(3) is {foo(3)}')
print(f'foo.y is {foo.y}')
print(f'foo.z is {foo.z}')

Output:
foo(3) is 6
foo.y is 2
foo.z is 8