Python Cookbook

Operating on Iterators

Credit: Sami Hangaslammi

Problem

You need to operate on iterators (including normal sequences) with the same semantics as normal sequence operations, except that lazy evaluation is a must, because some of the iterators involved could represent unbounded sequences.

Solution

Python 2.2 iterators are easy to handle via higher-order functions, and lazy evaluation (such as that performed by the xrange built-in function) can be generalized. Here are some elementary operations that include concatenating several iterators, terminating iteration when a function becomes false, terminating iteration after the first n values, and returning every nth result of an iterator:

from _ _future_ _ import generators

def itercat(*iterators):
    """ Concatenate several iterators into one. """
    for i in iterators:
        i = iter(i)
        for x in i:
            yield x

def iterwhile(func, iterator):
    """ Iterate for as long as func(value) returns true. """
    iterator = iter(iterator)
    while 1:
        next = iterator.next(  )
        if not func(next):
            raise StopIteration    # or: return
        yield next

def iterfirst(iterator, count=1):
    """ Iterate through 'count' first values. """
    iterator = iter(iterator)
    for i in xrange(count):
        yield iterator.next(  )

def iterstep(iterator, n):
    """ Iterate every nth value. """
    iterator = iter(iterator)
    while 1:
        yield iterator.next(  )
        # Skip n-1 values
        for dummy in range(n-1):
            iterator.next(  )

A bit less elementary, but still generally useful, are functions that transform an iterator’s output, not just selecting ...