How to create a generator function that calls generator functions

Question

Consider the following (non-working) example code:

class MyGenerator:
    def test_gen(self):
        for i in range(1,5):
            if i % 2:
                self.foo(i)
            else:
                self.bar(i)

    def foo(self, i):
        yield i

    def bar(self, i):
        yield i**2

g = MyGenerator()
for i in g.test_gen():
    print i

This will not work, because test_gen has no yield and is no longer a generator function. In this small example I could just return the values from foo and bar and put the yield into test_gen, however I have a case where that's not possible. How can I turn test_gen into a generator function again?

Martijn Pieters · Accepted Answer · 2014-06-26 11:43:30Z

5

You need to loop over the results of the delegated generators and yield those:

def test_gen(self):
    for i in range(1,5):
        if i % 2:
            for res in self.foo(i):
                yield res
        else:
            for res in self.bar(i):
                yield res

If you are using Python 3.3 or up, you'd use the yield from expression to do proper generator delegation:

def test_gen(self):
    for i in range(1,5):
        if i % 2:
            yield from self.foo(i)
        else:
            yield from self.bar(i)

Both re-introduce yield into the function, once again making it a generator function.

answered Jun 26, 2014 at 11:43

Martijn Pieters

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2 Comments

chiborg Over a year ago

Ha, the Python 3 way is exactly what I was looking for! Sweet and concise. Unfortunately I'm on Python 2.7 :-(

jsbueno Over a year ago

In this special example, the subgenerator will just yield a simple item before exhausting, so the yield from examples are fine. But people using it have to attempt they do yield up to exhaustion of the sub-generators, not a single item - and only after that the controls comes back to the primary generator. (otherwise they would not be distinguishable from a plain yield)

gkusner · Accepted Answer · 2014-06-26 13:33:13Z

0

why not just:

class MyGenerator:
    def test_gen(self):
        for i in range(1,5):
            if i % 2:
                yield next(self.foo(i))
            else:
                yield next(self.bar(i))

    def foo(self, i):
        yield i

    def bar(self, i):
        yield i**2

answered Jun 26, 2014 at 13:33

gkusner

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