Recursion using yield

Question

Is there any way to mix recursion and the yield statement? For instance, a infinite number generator (using recursion) would be something like:

def infinity(start):     yield start     # recursion here ...  >>> it = infinity(1) >>> next(it) 1 >>> next(it) 2 

I tried:

def infinity(start):     yield start     infinity(start + 1) 

and

def infinity(start):     yield start     yield infinity(start + 1) 

But none of them did what I want, the first one stopped after it yielded start and the second one yielded start, then the generator and then stopped.

NOTE: Please, I know you can do this using a while-loop:

def infinity(start):     while True:         yield start         start += 1 

I just want to know if this can be done recursively.

Answer 1

Yes, you can do this:

def infinity(start):     yield start     for x in infinity(start + 1):         yield x 

This will error out once the maximum recursion depth is reached, though.

Starting from Python 3.3, you'll be able to use

def infinity(start):     yield start     yield from infinity(start + 1) 

If you just call your generator function recursively without looping over it or yield from-ing it, all you do is build a new generator, without actually running the function body or yielding anything.

See PEP 380 for further details.