Iteration

[ibzi]

What is iteration in python.

i was learning new functions in python and crossed across these definitions

zip() Returns an iterator, from two or more iterators
iter() Returns an iterator object
map() Returns the specified iterator with the specified function applied to each item
reversed() Returns a reversed iterator

They used the word iterator and that, i don't know what it means . If you do i'd be extremely glad for your help. And while your there if you want to you can also help me with they definitions of these functions. Nevertheless, Thanks!

[buran]

https://www.python.org/dev/peps/pep-0234/
https://wiki.python.org/moin/Iterator

as to the functions - docs is always your best frend
https://docs.python.org/3/library/functions.html

[DeaD_EyE]

Repeating some calculation or something else is iteration.
The Iterator knows about it's own state/position.
An Iterator has the __next__ method, which is called by for-loops, lists, ... and everything
which consumes an iterable. Iterable does not mean, that this object is an Iterator.

class IterableIterator:
    def __init__(self):
        self.position = 0
        self.data = list(range(20))
    def __iter__(self):
        """
        When an object has the __next__ method, then
        it is an Iterable.
        """
        print('__iter__ called')
        return self

    def __next__(self):
        """
        If this method exists, then the object
        itself is an Iterator.
        """
        print('__next__ called')
        if self.position < 20:
            element = self.data[self.position]
            self.position += 1
            return element
        else:
            # have to raise an exception
            # as control flow for consumers
            raise StopIteration

class Iterator:
    def __init__(self, start, stop, step=1):
        self.start = start
        self.stop = stop
        self.step = step
        self.value = start
    def __next__(self):
        """
        When an object has the __next__ method, then
        it is an Iterator.
        """
        print('__next__ called')
        if self.value < self.stop:
            element = self.value
            self.value += self.step
            return element
        else:
            raise StopIteration

class Iterable:
    def __iter__(self):
        """
        This method is called by iter() or other
        built-ins. If there is an __iter__ method, then
        the object is Iterable
        """
        print('__iter__ called')
        return Iterator(0, 10, 1)

Output:
In [3]: it1 = IterableIterator()                                                                                                      

In [4]: iter(it1) == it1.__iter__()                                                                                                   
__iter__ called
__iter__ called
Out[4]: True

In [5]: list(it1)                                                                                                                     
__iter__ called
__next__ called
__next__ called
__next__ called
__next__ called
__next__ called
__next__ called
__next__ called
__next__ called
__next__ called
__next__ called
__next__ called
__next__ called
__next__ called
__next__ called
__next__ called
__next__ called
__next__ called
__next__ called
__next__ called
__next__ called
__next__ called
Out[5]: [0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19]

In [6]: list(it1)                                                                                                                     
__iter__ called
__next__ called
Out[6]: []

In [7]: # iterator has now been consumed. It's a finite iterator.                                                                     

In [8]: iterator_object = Iterator()                                                                                                  
---------------------------------------------------------------------------
TypeError                                 Traceback (most recent call last)
<ipython-input-8-0bd53d26c3d1> in <module>
----> 1 iterator_object = Iterator()

TypeError: __init__() missing 2 required positional arguments: 'start' and 'stop'

In [9]: iterator_object = Iterator(10, 13)                                                                                            

In [10]: next(iterator_object)                                                                                                        
__next__ called
Out[10]: 10

In [11]: next(iterator_object)                                                                                                        
__next__ called
Out[11]: 11

In [12]: next(iterator_object)                                                                                                        
__next__ called
Out[12]: 12

In [13]: next(iterator_object)                                                                                                        
__next__ called
---------------------------------------------------------------------------
StopIteration                             Traceback (most recent call last)
<ipython-input-13-c06eadb484b5> in <module>
----> 1 next(iterator_object)

<ipython-input-2-606497307f1e> in __next__(self)
     16             return element
     17         else:
---> 18             raise StopIteration
     19 
     20 class Iterable:

StopIteration: 

In [14]: iterable_but_not_iterator = Iterable()                                                                                       

In [15]: next(iterable_but_not_iterator)                                                                                              
---------------------------------------------------------------------------
TypeError                                 Traceback (most recent call last)
<ipython-input-15-173518d9b919> in <module>
----> 1 next(iterable_but_not_iterator)

TypeError: 'Iterable' object is not an iterator

In [16]: iterator_object2 = iter(iterable_but_not_iterator)                                                                           
__iter__ called

In [17]: iterator_object3 = iter(iterable_but_not_iterator)                                                                           
__iter__ called

In [18]: next(iterator_object2)                                                                                                       
__next__ called
Out[18]: 0

In [19]: next(iterator_object3)                                                                                                       
__next__ called
Out[19]: 0

In [20]: next(iterator_object2)                                                                                                       
__next__ called
Out[20]: 1

In [21]: next(iterator_object2)                                                                                                       
__next__ called
Out[21]: 2

In [22]: next(iterator_object3)                                                                                                       
__next__ called
Out[22]: 1

In [23]: # two independent iterators. This is because of the implementation 
    ...: # the class Iterable returns for each iter() call a new Iterator 
    ...: # The Iterator holds the state about the position 
    ...: # The Iterable returns an Iterator 
    ...: # Objects without a __iter__ and __next__ method are 
    ...: # still iterable, if they have a __len__ and __getitem__ method 
    ...: # Everywhere Iteration is used                                                                                               

In [24]: