Iteration, Iterable, Iterator and Generator

The object that is returned from range() function can be converted to a list, but actually, it just returns value one by one. range(1000000) does not preserve memory space for 1000000 integers. (range() function of Python2 returns List)

Any time you use a loop, explicit or implicit, to go over a group of items, that is iteration.

Iterable is an object that you can get an Iterator from. In other word, Iterable is an object that has an __iter__ method which returns Iterator. iter() function calls __iter__ method.

Iterator is an object with a __next__ method. next() function calls __next__ method.

The object such as String or List is an Iterable, but not Iterator, which can not return element to next().



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# iterable objecct retuns iterator to iter() function

s = 'abc'
itr = iter(s)

print(next(itr))
print(next(itr))
print(next(itr))



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## __iter__() function and __next__() function can be called directly as well

x = s.__iter__()
print(x.__next__())
print(next(x))



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# StopIteration signal is raised after all items are retrieved

print(next(itr))



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# STRING is a iterable, but not iterator. It does not support __next__ method

next(s)

Creating iterable object

Iterable object can be defined using class.



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class Reverse:
    """Iterator for looping over a sequence backwards."""
    def __init__(self, data):
        self.data = data
        self.index = len(data)

    def __iter__(self):
        return self

    def __next__(self):
        if self.index == 0:
            raise StopIteration
        self.index = self.index - 1
        return self.data[self.index]

rev = Reverse('UCSY')
for c in rev:
    print(c)
    
print('---')
    
rev = Reverse([100, 200, 300])
for c in rev:
    print(c)

Generator

Generators are a simple and powerful tool for creating iterators. They are written like regular functions but use the yield statement whenever they want to return data. Each time next() is called on it, the generator resumes where it left off (it remembers all the data values and which statement was last executed).



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def rev_gen(data):
#    for index in range(len(data)-1, -1, -1):
#        yield data[index]
    for elem in data[::-1]:
        yield elem

rev = rev_gen('ucsy')
for c in rev:
    print(c)



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type(rev)



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