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# Example of a generator: re-implement the built-in range function
# without actually constructing the list of values. (It turns out
# that the built-in function is about 20 times faster -- that's why
# it's built-in. :-)
# Wrapper function to emulate the complicated range() arguments
def range(*a):
if len(a) == 1:
start, stop, step = 0, a[0], 1
elif len(a) == 2:
start, stop = a
step = 1
elif len(a) == 3:
start, stop, step = a
else:
raise TypeError, 'range() needs 1-3 arguments'
return Range(start, stop, step)
# Class implementing a range object.
# To the user the instances feel like immutable sequences
# (and you can't concatenate or slice them)
class Range:
# initialization -- should be called only by range() above
def __init__(self, start, stop, step):
if step == 0:
raise ValueError, 'range() called with zero step'
self.start = start
self.stop = stop
self.step = step
self.len = max(0, int((self.stop - self.start) / self.step))
# implement `x` and is also used by print x
def __repr__(self):
return 'range' + `self.start, self.stop, self.step`
# implement len(x)
def __len__(self):
return self.len
# implement x[i]
def __getitem__(self, i):
if 0 <= i < self.len:
return self.start + self.step * i
else:
raise IndexError, 'range[i] index out of range'
# Small test program
def test():
import time, __builtin__
print range(10), range(-10, 10), range(0, 10, 2)
for i in range(100, -100, -10): print i,
print
t1 = time.millitimer()
for i in range(1000):
pass
t2 = time.millitimer()
for i in __builtin__.range(1000):
pass
t3 = time.millitimer()
print t2-t1, 'msec (class)'
print t3-t2, 'msec (built-in)'
test()
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