How yield catches StopIteration exception?

Question

Why in the example function terminates:

def func(iterable):
    while True:
        val = next(iterable)
        yield val

but if I take of

Answer 1

When a function contains yield, calling it does not actually execute anything, it merely creates a generator object. Only iterating over this object will execute the code. So my guess is that you're merely calling the function, which means the function doesn't raise StopIteration because it is never being executed.

Given your function, and an iterable:

def func(iterable):
    while True:
        val = next(iterable)
        yield val

iterable = iter([1, 2, 3])

This is the wrong way to call it:

func(iterable)

This is the right way:

for item in func(iterable):
    # do something with item

You could also store the generator in a variable and call next() on it (or iterate over it in some other way):

gen = func(iterable)
print(next(gen))   # prints 1
print(next(gen))   # prints 2
print(next(gen))   # prints 3
print(next(gen))   # StopIteration

By the way, a better way to write your function is as follows:

def func(iterable):
    for item in iterable:
        yield item

Or in Python 3.3 and later:

def func(iterable):
    yield from iter(iterable)

Of course, real generators are rarely so trivial. :-)