In this function y initially has no value, but a call to foo() gives no error. Why?

[Pedroski55]

This is from Reuven Lerner's book Python Workout.

I don't really understand why I don't get an error when no value is passed to y.
Or why, the second time around, I get the expected result.
(I added the print()s to show what is happening.

def foo(x):
    def bar(y):
        print(f'y is {y}')
        print(f'x is {x}')
        print(f'x * y is {x*y}')
        return x * y
    print(f'bar is {bar}')
    return bar

I call the function with g = foo(2), but y has no value. I expected an error, but I get：

g = foo(2) # returns: bar is <function foo.<locals>.bar at 0x7ff37b475268>

Why is there no error when y has no value??

Normally, if a function requires an argument and none is supplied, you will get an error.

Now I try:

print(g(20))

but I don't get:

foo(20) # returns: bar is <function foo.<locals>.bar at 0x7ff37b475268>

I get the intended result:

y is 20
x is 2
x * y is 40
40

How did 20 get passed to y??