Grabbing previous iteration in a dict

[Paul Hankin]

On May 9, 10:10 am, dannywebs... at googlemail.com wrote:
> Hello all,
>
> I have a dictionary of which i'm itervalues'ing through, and i'll be
> performing some logic on a particular iteration when a condition is
> met with trusty .startswith('foo').  I need to grab the previous
> iteration if this condition is met.  I can do something with an extra
> var to hold every iteration while iterating, but this is hacky and not
> elegant.

Often when you're iterating, 'hacky' code can be lifted out into a
separate generator, keeping the details of the hacks nicely away from
your code. That's true here...

def iterprevious(seq):
    """Generate pairs of (previous element, current element)
    from seq."""
    last = None
    for x in iter(seq):
        yield last, x
        last = x

Then, when you want use it...

for previous, (key, value) in iterprevious(d.iteritems()):
    ... In the loop, previous will either be None if we're on the
first element, otherwise (previous_key, previous_value).

--
Paul Hankin