eryksun added the comment: Refer to the documentation for deprecated __getslice__ when slicing an instance of a classic class:
https://docs.python.org/2/reference/datamodel.html#object.__getslice__ The SLICE+3 implementation (apply_slice) calls PySequence_GetSlice if both index values can be converted to Py_ssize_t integers and if the type defines sq_slice (instance_slice for the "instance" type). The "instance" type is used for an instance of a classic class. This predates unification of Python classes and types. apply_slice http://hg.python.org/cpython/file/f89216059edf/Python/ceval.c#l4383 PySequence_GetSlice http://hg.python.org/cpython/file/f89216059edf/Objects/abstract.c#l1995 instance_slice http://hg.python.org/cpython/file/f89216059edf/Objects/classobject.c#l1177 A new-style class, i.e. a class that subclasses object, would have to define or inherit __getslice__ in order for the C sq_slice slot to be defined. But __getslice__ is deprecated and shouldn't be implemented unless you have to override it in a subclass of a built-in type. When sq_slice doesn't exist, apply_slice instead calls PyObject_GetItem with a slice object: class A(object): def __getitem__(self, index): return index.start def __len__(self): return 10 >>> A()[-1:10] -1 By the way, you don't observe the behavior in Python 3 because it doesn't have classic classes, and the __getslice__, __setslice__, and __delslice__ methods are not in its data model. ---------- components: +Interpreter Core -Documentation nosy: +eryksun _______________________________________ Python tracker <rep...@bugs.python.org> <http://bugs.python.org/issue21785> _______________________________________ _______________________________________________ Python-bugs-list mailing list Unsubscribe: https://mail.python.org/mailman/options/python-bugs-list/archive%40mail-archive.com
