Function generators vs class generators in Python 3

Question

Why do function generators and class generators behave differently? I mean, with class generators I can use generator as many times as I want, but with function generators, I can only use it once? Why so?

def f_counter(low,high):
    counter=low
    while counter<=high:
        yield counter
        counter+=1

class CCounter(object):
    def __init__(self, low, high):
        self.low = low
        self.high = high
    def __iter__(self):
       counter = self.low
       while self.high >= counter:
            yield counter
            counter += 1

f_gen=f_counter(5,10)
for i in f_gen:
    print(i,end=' ')

print('\n')

for j in f_gen:
    print(j,end=' ')  #no output

print('\n')

c_gen=CCounter(5,10)
for i in c_gen:
    print(i,end=' ') 

print('\n')

for j in c_gen:
    print(j,end=' ')

Martijn Pieters · Accepted Answer · 2016-08-10 11:40:42Z

Calling the f_gen() function produces an iterator (specifically, a generator iterator). Iterators can only ever be looped over once. Your class is not an iterator, it is instead an iterable, an object that can produce any number of iterators.

Your class produces a new generator iterator each time you use for, because for applies the iter() function on the object you pass in, which in turn calls object.__iter__(), which in your implementation returns a new generator iterator each time it is called.

In other words, you can make the class behave the same way by calling iter(instance) or instance.__iter__() before looping:

c_gen = CCounter(5,10)
c_gen_iterator = iter(c_gen)
for i in c_gen_iterator:
    # ...

You can also make the CCounter() into an iterator by returning self from __iter__, and adding an object.__next__() method (object.next() in Python 2):

class CCounter(object):
    def __init__(self, low, high):
        self.low = low
        self.high = high
    def __iter__(self):
       return self
    def __next__(self):
       result = self.low
       if result >= self.high:
           raise StopIteration()
       self.low += 1
       return result

Blckknght · Accepted Answer · 2016-08-10 11:32:46Z

Your class is an iterable, but not an iterator itself. Each time you call iter on it, you get a new iterator.

If you want to replicate the behavior of the generator function with a class, then you want an iterator like this:

class CCounter(object):
    def __init__(self, low, high):
        self.low = low
        self.high = high
        self.counter = self.low
    def __iter__(self):
       return self
    def __next__(self):
       if self.counter > self.high:
           raise StopIteration()
       val = self.counter
       self.counter += 1
       return val

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