Method returning an Iterable Object

[Terry Reedy]

Anjanesh Lekshminarayanan wrote:
> But how come a raise StopIteration in the next() method doesnt need to
> be caught ? It works without breaking.

The for-loop looks for and catches StopIteration.  It is an essential 
part of what defines a finite iterator.
(Note, in 3.0, next is renamed __next__ in conformance with all other 
special methods.  In 2.6, the rename is optional, I believe.)

> class twoTimes:
>     max = 10**10
> 
>     def __init__(self, n):
>         self.__n = n

There is no reason to mangle the attribute name.
Max should be a parameter, and self.max = max added to the init.

> 
>     def next(self):
>         if self.__n > self.max:
>             raise StopIteration
>         self.__n *= 2
>         return self.__n

Are you sure you do not want to return once the initial value of n 
passed to init?
> 
>     def __iter__(self):
>         return self

Once you understand the above, you can rewrite it as a generator function:

def two_times(num, stop):
   while num < stop:
     yield num
     num *=2

The two last lines can be switches if desired.

> t = twoTimes(5)
> c = 0
> 
> print (t.next())
> print (t.next())
> 
> for n in t:
>     print n

 >>> print(list(two_times(5,687)))
[5, 10, 20, 40, 80, 160, 320, 640]

Terry Jan Reedy