What is xrange?

[DeaD_EyE]

I started to write a simple implementation and the result was this monster:

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from operator import lt, gt

def xrange(start, stop=None, step=1):
   """
   start is included
   stop is excluded
   step is default +1
   """
   if step == 0:
       raise ValueError('xrange() arg 3 must not be zero')
   op_cmp = lt if step > 0 else gt # get the right operator for comparison
   if stop is None:
       # in the case when you just use one argument
       # then the generator count up like the normal range function
       current = 0
       stop = start
   else:
       current = start
   while True:
       # stop is excluded
       if op_cmp(current, stop):
           yield current # first value is included
           current += step # then step
       else:
           break

But to understand it a bit better, here a stupid xrange:

import time

def xrange(stop):
   current = 0
   while True:
       if current < stop:
           yield current
           # the generator is suspended here
           # the state is saved in the generator
           current += 1
       else:
           break # stops the generator


generator = xrange(4)
print(next(generator))
time.sleep(1)
print(next(generator))
time.sleep(1)
print(next(generator))
time.sleep(1)
print(next(generator))
time.sleep(1)
print(next(generator))

Output:
0
1
2
3

Error:
StopIteration                             Traceback (most recent call last)
<ipython-input-53-793910a0cbc2> in <module>()
     8 print(next(generator))
     9 time.sleep(1)
---> 10 print(next(generator))

StopIteration:

The manual call with next is what a for-loop does automatically and when a StopIteration is raised, the for-loop stops. You won't see this error.
As mentioned before xrange has been renamed to range in Python 3.

If you want to generate an output which does not fit into your memory, you should use xrange in Python 2.
Then you can iterate over the xrange object and do stuff with the numbers.

If you need a list from a range function in Python 3, you have to call list(range(n)).