What is xrange?
steve+comp.lang.python at pearwood.info
Fri Jul 29 21:22:50 EDT 2011
Billy Mays wrote:
> Is xrange not a generator? I know it doesn't return a tuple or list, so
> what exactly is it?
xrange pre-dates generators by approximately forever. It returns a
special "xrange object", which generates values suitable for use in
for-loops using the old __getitem__ protocol.
You can consider xrange to be implemented something vaguely like this:
def __init__(self, start, end=None, step=1):
if end is None:
end = start
start = 0
self.start = start
self.end = end
self.step = step
def __getitem__(self, index):
if self.step != 1:
raise NotImplementedError('too lazy to support step values')
start, end = self.start, self.end
if start <= index < end:
return start + index
raise IndexError('out of range')
> Y doesn't ever complete, but x does.
> x = (i for i in range(10))
That is better written as iter(range(10)).
> y = xrange(10)
Y doesn't complete because xrange objects are restartable. The for-loop ends
up using the original iteration protocol:
i = y
i = y
i = y
until IndexError is raised. You break after calling y, but the next loop
starts at y again, and so you never progress beyond the first item in
the xrange object.
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