When I create a class based generator, why do I have to call the next? [duplicate]

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I'm new to Python and I'm reading the book Python Tricks. In the chapter about generators, it gives the following example (with some changes)

class BoundedGenerator:
def __init__(self, value, max_times):
self.value = value
self.max_times = max_times
self.count = 0

def __iter__(self):
return self

def __next__(self):
if self.count < self.max_times:
self.count += 1
yield self.value


After that, I write a loop, instantiate the generator and print the value:

for x in BoundedGenerator('Hello world', 4):
print(next(x))


Why do I have to call the next(X) inside the loop?

next(X)

I (think) I understand that the __iter__ function will be called in the loop line definition and the __next__ will be called in each iteration, but I don't understand why I have to call the next again inside the loop. Is this not redundant?
If I don't call the __next__ function, my loop will run forever.

__iter__

__next__

__next__

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__iter__

__next__

1 Answer
1

Your __next__ method itself is a generator function due to using yield. It must be a regular function that uses return instead.

__next__

yield

return

def __next__(self):
if self.count < self.max_times:
self.count += 1
return self.value # return to provide one value on call
raise StopIteration # raise to end iteration


When iterating, python calls iter.__next__ to receive the new value. If this is a generator function, the call merely returns a generator. This is the same behaviour as for any other generator function:

iter.__next__

>>> def foo():
... yield 1
...
>>> foo()
<generator object foo at 0x106134ca8>


This requires you to call next on the generator to actually get a value. Similarly, since you defined BoundedGenerator.__next__ as a generator function, each iteration step provides only a new generator.

next

BoundedGenerator.__next__

Using return instead of yield indeed returns the value, not a generator yielding said value. Additionally, you should raise StopIteration when done - this signals the end of the iteration.

return

yield

raise StopIteration