On Wednesday 20 April 2005 07:52 am, Antoon Pardon wrote: > Personnaly I would like to have the choice. Sometimes I prefer to > start at 0, sometimes at 1 and other times at -13 or +7.
Although I would classify that as a "rare use case". So, it "ought to be possible to do it, but not necessarily easy". Which Python obliges you on --- you can easily create a relocatable list class that indexes and slices anyway you see fit, e.g.: >>> class reloc(list): ... def __init__(self, start, contents=()): ... self.start = start ... for item in contents: ... self.append(item) ... def __getitem__(self, index): ... if isinstance(index, slice): ... if (not (self.start <= index.start < self.start + len(self)) or ... not (self.start <= index.stop < self.start + len(self))): ... raise IndexError ... return self.__class__.__base__.__getitem__(self, slice(index.start-self.start, index.stop-self.start, index.step)) ... else: ... if not (self.start <= index < self.start + len(self)): ... raise IndexError ... return self.__class__.__base__.__getitem__(self, index - self.start) ... ... >>> >>> r = reloc(-13, ['a', 'b', 'c', 'd', 'e', 'f', 'g', 'h', 'i', 'j', 'k', 'l', >>> 'm', 'n', 'o', 'p', 'q']) >>> r[0] 'n' >>> r[-12] 'b' >>> r[-13] 'a' >>> r[-14] Traceback (most recent call last): File "<stdin>", line 1, in ? File "<stdin>", line 14, in __getitem__ IndexError >>> r[0] 'n' >>> r[3] 'q' >>> r[4] Traceback (most recent call last): File "<stdin>", line 1, in ? File "<stdin>", line 14, in __getitem__ IndexError >>> Note that I added the IndexError to avoid the bizarre results you get due to "negative indexing" in the list base class (now they would change behavior at -13 instead of at 0, which is counter- intuitive to say the least). Better to cry foul if an index out of range is called for in this case, because it's gotta be a bug. What do you think, should I send it to Useless Python? ;-) Cheers, Terry -- Terry Hancock ( hancock at anansispaceworks.com ) Anansi Spaceworks http://www.anansispaceworks.com -- http://mail.python.org/mailman/listinfo/python-list
