• RangeGenerator

Below is a little range generator, irange, which is compatible with range. More specifically, [i for i in irange(*args)] == range(*args)]. This will let us iterator over large spans of numbers without resorting to xrange, which is a lazy list as opposed to a generator.

Would a function of similar semantics likely be accepted into the standard library within itertools?

lwickjr: I like the idea. Anyone else?

The author doesn't have the time/energy to write/push a PEP for the PythonEnhancementProcess. If you think this generator is a good idea, please submit a PEP.

lwickjr: Neither do I. Anyone else?

Test Suite

Here is the test:

import unittest

from iter import irange
class TestIter(unittest.TestCase):
    def testIrangeValues(self):
        testArguments = ( [10],[0],[7,70], [-10], [10,700,17], [10,1000,-3],[10,-99,-7] )
        l = lambda *x: list(irange(*x))

        for args  in testArguments:
            list1 = l(*args)
            list2 = range(*args)
            self.assertEqual(list1,list2)

    def testIrangeArguments(self):
         self.assertRaises(ValueError,irange,0,99,0) #0 step
        #self.assertRaises(irange(1,2,3,4)) #too many arguments



if __name__ == '__main__':
    unittest.main()

implementation

"""generator for a range of integers that matches the output
of an iterator over the list range(...)

"""

from itertools import count,takewhile
"""private generator over the unchecked arguments start, stop, step
   [i for i in __irange3(start,stop,step)] produces range(start,stop,step)
"""
def __irange3__(start,stop,step):
    #the stop condition changes depending on the sign of step
    predicate = step > 0 and (lambda x : x < stop) or (lambda x : x > stop)
    i = start
    while predicate(i):
        yield i
        i += step

""" generator to produce the same output as an iterator over
range(args).  The advantage of this over range() or xrange() is a list
is not created in order to provide the generator.
[i for i in irange(args) ] == [range(args)]
"""
def irange(*args):
    if len(args) not in (1,2,3):
        raise TypeError, "expected 1,2, or 3 arguments to irange"

    #the stop value will be the 1st argument when 1 argument supplied, it
    #will the the second argument otherwise. The index is one less than the
    #argument
    stop = args[(1,0)[len(args) == 1]]
    if len(args) == 3:
        step = args[2]
        #we check the step before we create the generator, for earlier
        #error annunciation.
        if step == 0:
            raise ValueError()
        return __irange3__(args[0],stop,step)

    #for the cases with no step (1 or two args) its easy to
    #use the built in count() generator and filter
    predicate = lambda x : x < stop
    #set up the arguments to count, depending on whether we have a start
    counter_args = len(args) != 1 and [args[0]] or ()
    counter = count(*counter_args)
    return takewhile(predicate, counter)

Alternate Implementation

Perhaps a simple implementation can be constructed using *count* and *islice* from intertools?. -- Anon

I think it's way too much code, and also it does not accept named parameters. How about this? -- JürgenHermann 2005-10-19 20:06:52

• def irange(start, stop=None, step=1):
      """ Generator for a list containing an arithmetic progression of integers.
          range(i, j) returns [i, i+1, i+2, ..., j-1]; start (!) defaults to 0.
          When step is given, it specifies the increment (or decrement).
          For example, range(4) returns [0, 1, 2, 3].  The end point is omitted!
          These are exactly the valid indices for a list of 4 elements.
      """
      if step == 0:
          raise ValueError("irange() step argument must not be zero")
  
      if stop is None:
          stop = start
          start = 0
      continue_cmp = (step < 0) * 2 - 1
  
      while cmp(start, stop) == continue_cmp:
          yield start
          start += step
  
  if __name__ == "__main__":
      cases = [
          (0,),
          (1,),
          (2,),
  
          (1, 0),
          (1, 1),
          (1, 2),
          (1, 4),
  
          (1, 4, 1),
          (1, 4, 2),
          (1, 4, 3),
          (1, 4, -1),
          (1, 4, -2),
          (1, 4, -3),
  
          (4, 1, 1),
          (4, 1, 2),
          (4, 1, 3),
          (4, 1, -1),
          (4, 1, -2),
          (4, 1, -3),
      ]
      for case in cases:
          assert range(*case) == [i for i in irange(*case)]
  
      print "All %d tests passed!" % len(cases)

The alternate implementation is fine, though I'd prefer to see a "takewhile" rather than a while loop in the spirit of functional programming - but thats minor.

The test case I am less fond of - while it does test the functionality, it doesn't support TestDrivenDevelopment as well. It would be nice to have a test from unitest to allow someone building a big system to easily run a suite of tests they like.

Next step write a PEP someone?