Iterators and iterables.

1. Definitions

An iterator is an object that contains a countable number of values that can be iterated, meaning that it is possible to traverse through all the values. In Python, an iterator is a object whose class implements the __next__() and __iter__() methods, in the following way:



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class yrange():
    
    def __init__(self, max_i):
        self.i = 0
        self.max_i = max_i
        
    def __iter__(self):
        return self # Compulsory when implementing the iterator protocol
    
    def __next__(self):
        if self.i > self.max_i:
            raise StopIteration # Compulsory (for finite iterators) when implementing the iterator protocol
        else:
            self.i += 1
            return self.i - 1



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iterator = yrange(5)
next(iterator)



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next(iterator)



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iterator.__next__() # This is also possible



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next(iterator)



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next(iterator)



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next(iterator)

Iterators are commonly used in for structures:



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for i in yrange(10):
    print(i, end=' ')

An iterable is an object which one can iterate over. All iterators are iterable, but not all iterable objects are iterators. For example, a list is iterable and it not an iterator, unless it is converted in an iterator using the method iter():



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i = iter(['a', 'b', 'c']) # A list
i.__next__()



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i.__next__()



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i.__next__()



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i.__next__()

3. Iterators everywhere

Most of the native containers in Python can be populated with iterators:



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list(range(10))



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tuple(range(3,9))

Strings also support the iterator protocol:



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set('hola')

Be aware!

Iterators spend memory.



In [1]:

    
list_of_squares = [x*x for x in range(3)]



In [2]:

    
list_of_squares









    Out[2]:





[0, 1, 4]