>>> for n in xrange(5):
...     print n
... 
0
1
2
3
4
>>> list(xrange(3))
[0, 1, 2]
>>> [n**2 for n in xrange(4)]
[0, 1, 4, 9]

# this requires 4 MB to run:
sumOfSquares=0
for n in range(1000000):
   sumOfSquares+=n*n
 
# this requires 8 bytes to run:
sumOfSquares=0
for n in xrange(1000000):
   sumOfSquares+=n*n

(Jun-01-2017, 09:50 AM)Ofnuts Wrote: In python2 xrange(..) creates the values when they are needed (ie, when your code asks for the next value...), while range() generates all the possible values at once. No big difference when you are using small ranges, but vital if you are using large ranges, especially if your logic makes it unlikely that you use up all the generated values.

# this requires 4 MB to run:
sumOfSquares=0
for n in range(1000000):
   sumOfSquares+=n*n
 
# this requires 8 bytes to run:
sumOfSquares=0
for n in xrange(1000000):
   sumOfSquares+=n*n

>>> class xrange:
...   def __init__(self, min=0, max=None, step=1):
...     if max is None:
...       max = min
...       min = 0
...     self.max = max
...     self.step = step
...     self.current = min
...   def __iter__(self):
...     return self
...   def __next__(self):
...     value = self.current
...     self.current += self.step
...     if self.step > 0:
...       if value >= self.max:
...         raise StopIteration()
...     else:
...       if value <= self.max:
...         raise StopIteration()
...     return value
...
>>> for i in xrange(5):
...   print(i)
...
0
1
2
3
4
>>> for i in xrange(2, 12, 3):
...   print(i)
...
2
5
8
11

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

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: