Is it possible to mimic a python slice object or subclass?

Question

I would like to include some metadata into a python slice object, along with adding variables to indicate the index of each element in the slice. The metadata is used to label each element that the slice is retrieving. I know there are other labelled data structures that can be used, but in my project slices are predefined as a sort of subscript for numpy arrays and is re-used in various places. So, for me it makes sense to find a way to incorporate this.

I was thinking of sub-classing slice, but apparently it cannot be subclassed which was explained clearly in the answer of the linked question. Has anything changed since then?

What I'd like to do is create a class that looks like:

class Subscript:
    def __init__(self, start, stop, step=None, labels=None):
        self.labels = labels
        self.slc = slice(start, stop, step)

        for i, l in zip(range(start, stop, step), labels):
            setattr(self, l, i)

and be able to use it like this:

sub = Subscript(0, 5, labels=['s0', 's1', 's2', 's3', 's4'])

list(range(10))[sub]  # [0, 1, 2, 3, 4]

range(10)[sub.s0]  # 0

is there a way to do this without having to add a __call__ method to return the slice? Somehow I doubt this because the array or list taking in the sub through __getitem__ wouldn't know what to do with this. I know that I could probably just monkey-patch this information to slice, but am wondering if this type of thing could be done in a class.

Currently, I am defining the slice and slice elements separately like:

sub = slice(0, 5)

s0, s1, s2, s3, s4 = range(5)

But this approach makes it much harder to process the output of multidimensional arrays into a dict where keys are subscript element combinations in the case of more than 1 sub and values are 1d arrays.

Please don't use `exec` to set the attributes dynamically, `setattr` is much better suited for this. Also, your `__init__` doesn't have `self`. — vaultah, Nov 23 '16 at 15:51

Dimitris Fasarakis Hilliard · Answer 1 · 2016-11-23T20:58:08.313

1

Nope, slice objects still can't be sub-classed. I'm saying this based on the flags defined for PySlice_Type in the default Python (3.7) branch:

Py_TPFLAGS_DEFAULT | Py_TPFLAGS_HAVE_GC,    /* tp_flags */

To allow an object to act as a base class the appropriate Py_TPFLAGS_BASETYPE would be ored in there as they are with types defined allowed to. Taking lists as an example, their flags are defined as:

Py_TPFLAGS_DEFAULT | Py_TPFLAGS_HAVE_GC |
    Py_TPFLAGS_BASETYPE | Py_TPFLAGS_LIST_SUBCLASS,         /* tp_flags */

Ignoring the rest, Py_TPFLAGS_BASETYPE is |'ed in there allowing it to act as a base class.

Judging by the fact that I couldn't find this mentioned somewhere in the docs, I'd say it's an implementation detail whose rationale I'm currently not aware of. The only way I believe you might circumvent it is by dropping to C and making your class there.

edited Nov 23 '16 at 20:58

answered Nov 23 '16 at 19:00

Dimitris Fasarakis Hilliard

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Thanks for this. Looks like this is trickier than I thought. Even monkey-patching `slice` doesn't seem to be do-able because of this. I just get an `AttributeError` instead. – pbreach Nov 24 '16 at 15:58
1

Yup @pbreach if I remember correctly, `list.__getitem__` (and the other containers should behave similarly) checks explicitly for a `slice` type as the argument passed to it or `int` values (at least, objects that provide an `__index__` method that returns an appropriate `int`). The best you'd likely be able to do is actually create a new `list` subtype that allows for custom objects that behave like `slice`s (by composition maybe?). – Dimitris Fasarakis Hilliard Nov 24 '16 at 16:18
I ended up doing basically what you suggested except by subclassing `numpy.ndarray` instead of `list` and posted it here. Works for me! – pbreach Nov 24 '16 at 22:48

pbreach · Answer 2 · 2016-11-24T23:42:21.243

What I ended up doing is subclassed numpy.ndarray because I was only trying to pass the slices into this type of object (could do the same for list), and then reimplemented __getitem__ so that if a Subscript object is passed in then the slice will first be extracted before passing onto the parent method.

Looks like:

import numpy as np

class SubArray(np.ndarray):
    def __new__(cls, input_array, subs=None):
        obj = np.asarray(input_array).view(cls)
        obj.subs = subs
        return obj

    def __getitem__(self, *args):
        args = tuple([a.slc if isinstance(a, SubRange) else a for a in args])
        return super().__getitem__(*args)

    def __array_finalize__(self, obj):
        if obj is None:
            return
        self.subs = getattr(obj, 'subs', None)


class Subscript:
    def __init__(self, labels, bounds=None):
        name, elements = labels

        if bounds:
            start, stop = bounds
        else:
            start, stop = 0, len(elements)

        self.size = stop - start
        self.slc = slice(start, stop)
        self.labels = labels
        self.name = name
        self.elements = elements

        for l, i in zip(labels, range(start, stop)):
            setattr(self, l, i)

And can use like this:

sub = Subscript(('sub', ['s0', 's1', 's2', 's3', 's4']))

SubArray(np.arange(10), subs=sub)[sub]  # SubArray([0, 1, 2, 3, 4])

SubArray(np.arange(10), subs=sub)[sub.s0]  # 0

This is much closer to the approach that I was avoiding (i.e. using something like xarray), but the result is still basically a numpy array and works for me.

Is it possible to mimic a python slice object or subclass?

2 Answers2