Allow us to present a generator function for the Lucas series:

def lucas():
    yield 2
    a = 2
    b = 1
    while True:
        yield b
        a, b = b, a + b

The Lucas series starts with 2, 1, and each value after that is the sum of the two preceding values. So the first few value of the sequence are:

2, 1, 3, 4, 7, 11

The first yield produces the value 2. The function then initializes a and b which hold the "previous two values" needed as the function proceeds. Then the function enters an infinite while-loop where:

1. It yields the value of b
2. a and b are updated to hold the new "previous two" values using a neat application of tuple unpacking

Now that we have a generator, it can be used like any other iterable object. For instance, to print the Lucas numbers you could use a loop like this:

>>> for x in lucas():
...     print(x)
...
2
1
3
4
7
11
18
29
47
76
123
199

Of course, since the Lucas sequence is infinite this will run forever, printing out values until your computer runs out of memory. Use Ctrl+C to terminate the loop.