Make `copy.deepcopy` faster for `slice` objects

[sobolevn]

slice

Right now slice objects are deepcopied using __reduce__ defined as:

cpython/Objects/sliceobject.c

Lines 559 to 563 in 26ff436

	static PyObject *
	slice_reduce(PySliceObject* self, PyObject *Py_UNUSED(ignored))
	{
	return Py_BuildValue("O(OOO)", Py_TYPE(self), self->start, self->stop, self->step);
	}

It is one of last branches of deepcopy logic:

cpython/Lib/copy.py

Lines 120 to 157 in 26ff436

	def deepcopy(x, memo=None, _nil=[]):
	"""Deep copy operation on arbitrary Python objects.
	See the module's __doc__ string for more info.
	"""
	if memo is None:
	memo = {}
	d = id(x)
	y = memo.get(d, _nil)
	if y is not _nil:
	return y
	cls = type(x)
	copier = _deepcopy_dispatch.get(cls)
	if copier is not None:
	y = copier(x, memo)
	else:
	if issubclass(cls, type):
	y = _deepcopy_atomic(x, memo)
	else:
	copier = getattr(x, "__deepcopy__", None)
	if copier is not None:
	y = copier(memo)
	else:
	reductor = dispatch_table.get(cls)
	if reductor:
	rv = reductor(x)
	else:
	reductor = getattr(x, "__reduce_ex__", None)
	if reductor is not None:
	rv = reductor(4)
	else:
	reductor = getattr(x, "__reduce__", None)
	if reductor:
	rv = reductor()

But, since slice is an immutable type without nested structures, we can optimize its deepcopy as:

d[slice] = _deepcopy_atomic

Before:

» pyperf timeit --setup 'from copy import deepcopy; s = slice(1,10,2)' 'deepcopy(s)'
.....................
Mean +- std dev: 3.46 us +- 0.18 us

After:

» pyperf timeit --setup 'from copy import deepcopy; s = slice(1,10,2)' 'deepcopy(s)'
.....................
Mean +- std dev: 277 ns +- 3 ns

Looks like a good speedup for just a single line!
Noticed while working on #100815

PR is incoming.

Linked PRs

• gh-100817: Speed up copy.deepcopy calls on slice objects #100818

[sobolevn]

Closing, see #100818

[eendebakpt]

@sobolevn PR #100817 makes the deepcopy faster, which also has a positive effect on the speed of the slice deep copy:

Main:

$ ./python -m pyperf timeit --setup 'from copy import deepcopy; s = slice(1,10,2)' 'deepcopy(s)'
.....................
Mean +- std dev: 2.96 us +- 0.05 us

PR 100817

 $ ./python -m pyperf timeit --setup 'from copy import deepcopy; s = slice(1,10,2)' 'deepcopy(s)'
.....................
Mean +- std dev: 2.47 us +- 0.07 us