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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()