The main issue I’ve had with a “Slicer” class is that it feels like this should belong to slice itself. But I’ve finally landed on a solution I like that lets you create something that looks a whole lot like a Slice subclass:
class _SliceMeta(type):
"""
Normally you cannot inherit from slice, but we can use a metaclass to
overload the constructor of our Slice class to return a real slice.
"""
def __call__(cls, *args):
# Allow for empty constructor
if len(args) == 0:
return slice(None)
# When the class is called (instantiated), return a real slice instead
return slice(*args)
def __instancecheck__(self, instance):
# https://peps.python.org/pep-3119/
return isinstance(instance, slice)
def __getitem__(self, index):
# __class_getitem__ was introduced in Python 3.7
# https://peps.python.org/pep-0560/
return index
class Slice(metaclass=_SliceMeta):
"""
A "subclass" of :class:`slice` with convinience features.
Namely you can create a slice with class getitem syntax.
The motiviation is creating explicit slice objects is cumbersome, and this
provides a more concise notation. Consider the example
.. code:: python
import kwarray
# This is the standard way to create multi dimensional index
sl = (Ellipsis, slice(0, 10), slice(0, 10), slice(None))
arr = arr[sl]
# And this is equivalent
sl = kwarray.Slice[..., 0:10, 0:10, :]
arr = arr[sl]
Note:
This is not a real subclass of slice, but by using metaclasses it
behaves like one in most circumstances. The instances created by using
the constructor or convinience methods are real slice instances (or
tuples of real slice instances).
A similar idea has been proposed and rejected several times to core Python
[CPythonIssue68567]_ [PythonDiscuss30316]_.
References:
.. [PythonDiscuss30316] https://discuss.python.org/t/alternative-to-create-slice-and-tuple-of-slices-with-slice-class-getitem/30316/6
.. [CPythonIssue68567] https://github.com/python/cpython/issues/68567
Example:
>>> import kwarray
>>> kwarray.Slice[::3]
slice(None, None, 3)
>>> kwarray.Slice[0:10, 3:11]
(slice(0, 10, None), slice(3, 11, None))
Example:
>>> from kwarray.util_slices import Slice
>>> assert Slice() == slice(None, None, None)
>>> assert Slice[0:3] == slice(0, 3, None)
>>> assert Slice[0:3, 0:5] == (slice(0, 3, None), slice(0, 5, None))
>>> assert Slice[..., 0:3, 0:5] == (slice(0, 3, None), slice(0, 5, None))
>>> assert Slice(0, 3) == (slice(0, 3, None), slice(0, 5, None))
>>> assert isinstance(Slice(), Slice)
>>> assert not issubclass(Slice, slice), 'no way to make this work AFAIK'
"""
def __init__(self, *args):
raise AssertionError('It should not be possible to construct a real instance of this class')