[clug] Two level python iterator

[Hugh Fisher]

jm wrote:
> One for the python developers out there.
> 
> I have a class which stores data in a hash of a hash and I which I wish 
> to iterate over all the entries. Something like that shown below. What 
> do I write for the __iter__ and next methods so that I can loop over the 
> data with one for loop? Been thinking in languages other than python 
> lately and just can't see the answer at the moment.
> 
> class TwoLevel:
> ....
>    def add_data(self, idx1, idx2, value):
>      self.data[idx1][idx2] = value
> 
>   def __iter__(self):
>      # what goes here
> 
>   def next(self):
>      # and here
> 
> 

You could use the itertools module, which creates iterators over
multiple iterable objects. (I'm assuming you use lists or something
for the levels of your data structure.)

import itertools

def __iter__ (self):
	return itertools.chain.from_iterable(self.data)

# No next(self) required


If you need to control the order a bit more precisely or munge your
own data structures, the code is something like:

def __iter__ (self):
	return self	# Object itself implements next()

def next (self):
	for outer in self.data:
		for inner in outer:
			yield inner
	raise StopIteration


Hope this helps.

	cheers,
	Hugh Fisher