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"""Example of a generator: re-implement the built-in range function
without actually constructing the list of values.
OldStyleRange is coded in the way required to work in a 'for' loop before
iterators were introduced into the language; using __getitem__ and __len__ .
"""
def handleargs(arglist):
"""Take list of arguments and extract/create proper start, stop, and step
values and return in a tuple"""
try:
if len(arglist) == 1:
return 0, int(arglist[0]), 1
elif len(arglist) == 2:
return int(arglist[0]), int(arglist[1]), 1
elif len(arglist) == 3:
if arglist[2] == 0:
raise ValueError("step argument must not be zero")
return tuple(int(x) for x in arglist)
else:
raise TypeError("range() accepts 1-3 arguments, given", len(arglist))
except TypeError:
raise TypeError("range() arguments must be numbers or strings "
"representing numbers")
def genrange(*a):
"""Function to implement 'range' as a generator"""
start, stop, step = handleargs(a)
value = start
while value < stop:
yield value
value += step
class oldrange:
"""Class implementing a range object.
To the user the instances feel like immutable sequences
(and you can't concatenate or slice them)
Done using the old way (pre-iterators; __len__ and __getitem__) to have an
object be used by a 'for' loop.
"""
def __init__(self, *a):
""" Initialize start, stop, and step values along with calculating the
nubmer of values (what __len__ will return) in the range"""
self.start, self.stop, self.step = handleargs(a)
self.len = max(0, (self.stop - self.start) // self.step)
def __repr__(self):
"""implement repr(x) which is also used by print"""
return 'range(%r, %r, %r)' % (self.start, self.stop, self.step)
def __len__(self):
"""implement len(x)"""
return self.len
def __getitem__(self, i):
"""implement x[i]"""
if 0 <= i <= self.len:
return self.start + self.step * i
else:
raise IndexError('range[i] index out of range')
def test():
import time, __builtin__
#Just a quick sanity check
correct_result = __builtin__.range(5, 100, 3)
oldrange_result = list(oldrange(5, 100, 3))
genrange_result = list(genrange(5, 100, 3))
if genrange_result != correct_result or oldrange_result != correct_result:
raise Exception("error in implementation:\ncorrect = %s"
"\nold-style = %s\ngenerator = %s" %
(correct_result, oldrange_result, genrange_result))
print("Timings for range(1000):")
t1 = time.time()
for i in oldrange(1000):
pass
t2 = time.time()
for i in genrange(1000):
pass
t3 = time.time()
for i in __builtin__.range(1000):
pass
t4 = time.time()
print(t2-t1, 'sec (old-style class)')
print(t3-t2, 'sec (generator)')
print(t4-t3, 'sec (built-in)')
if __name__ == '__main__':
test()
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