TIL #010 – generator return

Generators

Generators are interesting Python objects that produce a series of values, but one by one. In a way, they can be thought of as stateful functions. (That is, functions with state.)

What I learned is that generators can also return something. Here is the tweet that prompted this discovery:

#Python pop quiz: Is this code valid? If so, what does it do?

def f():
yield 10
return 20

g = f()
print(next(g))
print(next(g))

— Raymond Hettinger (@raymondh) October 7, 2021

Returning from a generator

When you use a return inside a generator, the generator will have that returned information in its StopIteration exception when it's done:

>>> def f():
...     yield 10
...     return 20
...
>>> gen = f()
>>> next(gen)
10
>>> next(gen)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
StopIteration: 20

If you want to get access to that value, you just need to catch the exception:

>>> gen = f(); next(gen);
10
>>> try:
...     next(gen)
... except StopIteration as e:
...     val = e.value
...
>>> val
20

That's it for now! Stay tuned and I'll see you around!

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