Help on for loop understanding

[Robert]

On Monday, December 7, 2015 at 8:39:48 PM UTC-5, Robert wrote:
> On Monday, December 7, 2015 at 8:32:30 PM UTC-5, Robert wrote:
> > On Monday, December 7, 2015 at 8:14:46 PM UTC-5, Robin Koch wrote:
> > > Am 08.12.2015 um 02:05 schrieb Robert:
> > > > Hi,
> > > > When I learn for loop with below link:
> > > >
> > > > http://www.shutupandship.com/2012/01/understanding-python-iterables-and.html
> > > >
> > > > it has such explanation:
> > > >
> > > > \\\\\\\\\
> > > > for loop under the hood
> > > >
> > > > First let's look at the for loop under the hood. When Python executes the
> > > >   for loop, it first invokes the __iter__() method of the container to get the
> > > >   iterator of the container. It then repeatedly calls the next() method
> > > >   (__next__() method in Python 3.x) of the iterator until the iterator raises a
> > > >   StopIteration exception. Once the exception is raised, the for loop ends.
> > > > \\\\\\\\\
> > > >
> > > > When I follow a list example from the above link, and one example of myself:
> > > >
> > > > //////////
> > > > xx=[1,2,3,4,5]
> > > >
> > > > xx.next
> > > > ---------------------------------------------------------------------------
> > > > AttributeError                            Traceback (most recent call last)
> > > > <ipython-input-76-dd0716c641b1> in <module>()
> > > > ----> 1 xx.next
> > > >
> > > > AttributeError: 'list' object has no attribute 'next'
> > > >
> > > > xx.__iter__
> > > > Out[77]: <method-wrapper '__iter__' of list object at 0x000000000A1ACE08>
> > > >
> > > > for c in xx: print c
> > > > 1
> > > > 2
> > > > 3
> > > > 4
> > > > 5
> > > > //////////////
> > > >
> > > > I am puzzled that the list examples have no next method, but it can run as
> > > > a for loop. Could you explain it to me the difference?
> > > 
> > > Lists don't have a next method. Their iterators have:
> > > 
> > > xx.__iter__().__next__()
> > > 
> > > or
> > > 
> > > xxIterator = xx.__iter__()
> > > xxIterator.__next__()
> > > xxIterator.__next__()
> > > xxIterator.__next__()
> > > xxIterator.__next__()
> > > xxIterator.__next__()
> > > 
> > > That's also what your quoted paragraph states:
> > > 
> > > | It then repeatedly calls the next() method
> > > | (__next__() method in Python 3.x) of the iterator
> > > 
> > > -- 
> > > Robin Koch
> > 
> > I use Python 2.7. I have tried these commands:
> > 
> > xx=[1,2,3,4,5]
> > 
> > xx.__iter__
> > Out[2]: <method-wrapper '__iter__' of list object at 0x000000000A1B85C8>
> > 
> > xx.__iter__().__next__()
> > ---------------------------------------------------------------------------
> > AttributeError                            Traceback (most recent call last)
> > <ipython-input-3-980bcf1bbf42> in <module>()
> > ----> 1 xx.__iter__().__next__()
> > 
> > AttributeError: 'listiterator' object has no attribute '__next__' 
> > 
> > xx.__iter__.__next__
> > ---------------------------------------------------------------------------
> > AttributeError                            Traceback (most recent call last)
> > <ipython-input-4-df8d8c6955e2> in <module>()
> > ----> 1 xx.__iter__.__next__
> > 
> > AttributeError: 'method-wrapper' object has no attribute '__next__' 
> > 
> > xx.__iter__()
> > Out[5]: <listiterator at 0xa211780>
> > 
> > xx.__iter__().__next__
> > ---------------------------------------------------------------------------
> > AttributeError                            Traceback (most recent call last)
> > <ipython-input-6-92f21313d62e> in <module>()
> > ----> 1 xx.__iter__().__next__
> > 
> > AttributeError: 'listiterator' object has no attribute '__next__' 
> > 
> > xxIterator = xx.__iter__()
> > 
> > xxIterator
> > Out[8]: <listiterator at 0xa2115c0>
> > 
> > xxIterator.__next__()
> > ---------------------------------------------------------------------------
> > AttributeError                            Traceback (most recent call last)
> > <ipython-input-9-5b74e35c2c6e> in <module>()
> > ----> 1 xxIterator.__next__()
> > 
> > AttributeError: 'listiterator' object has no attribute '__next__' 
> > 
> > for c in xx: print c
> > 1
> > 2
> > 3
> > 4
> > 5
> > ------------
> > 
> > I don't find a way to show __next__ yet. 
> > Can we explicitly get the iterator for a list?
> > Thanks,
> 
> Excuse me. I find it as the following:
> 
> xx.__iter__().next
> Out[16]: <method-wrapper 'next' of listiterator object at 0x0000000008B38AC8>
> 
> xx.__iter__().next()
> Out[17]: 1

One example, see below please, is in the above mentioned link. I don't see
the purpose of the example.


\\\\\\\\\\\
class MyList(list):
    def __iter__(self):
        return MyListIter(self)
    
class MyListIter(object):
    """ A sample implementation of a list iterator. NOTE: This is just a 
    demonstration of concept!!! YOU SHOULD NEVER IMPLEMENT SOMETHING LIKE THIS!
    Even if you have to (for any reason), there are many better ways to 
    implement this."""
    def __init__(self, lst):
        self.lst = lst
        self.i = -1
    def __iter__(self):
        return self
    def next(self):
        if self.i<len(self.lst)-1:
            self.i += 1         
            return self.lst[self.i]
        else:
            raise StopIteration

if __name__ == '__main__':
    a = MyList([1, 2, 3, 4])
    ia = iter(a)
    print 'type(a): %r, type(ia): %r' %(type(a), type(ia))
    for i in a: 
        print i,
\\\\\\\\\\\\\\\

I have the following two same results. What do they tell me? Thanks,

for c in a: print c
1
2
3
4

for c in ia: print c
1
2
3
4