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[3.9] Clarify that Set._from_iterable is not required to be a classmethod. (GH-23272) #23450

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Merged
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Nov 21, 2020
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[3.9] Clarify that Set._from_iterable is not required to be a classmethod. (GH-23272) #23450

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2 changes: 1 addition & 1 deletion Doc/library/collections.abc.rst
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Original file line number Diff line number Diff line change
Expand Up @@ -291,7 +291,7 @@ Notes on using :class:`Set` and :class:`MutableSet` as a mixin:
:meth:`_from_iterable` which calls ``cls(iterable)`` to produce a new set.
If the :class:`Set` mixin is being used in a class with a different
constructor signature, you will need to override :meth:`_from_iterable`
with a classmethod that can construct new instances from
with a classmethod or regular method that can construct new instances from
an iterable argument.

(2)
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56 changes: 56 additions & 0 deletions Lib/test/test_collections.py
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Expand Up @@ -1558,6 +1558,62 @@ def assertSameSet(self, s1, s2):
# coerce both to a real set then check equality
self.assertSetEqual(set(s1), set(s2))

def test_Set_from_iterable(self):
"""Verify _from_iterable overriden to an instance method works."""
class SetUsingInstanceFromIterable(MutableSet):
def __init__(self, values, created_by):
if not created_by:
raise ValueError(f'created_by must be specified')
self.created_by = created_by
self._values = set(values)

def _from_iterable(self, values):
return type(self)(values, 'from_iterable')

def __contains__(self, value):
return value in self._values

def __iter__(self):
yield from self._values

def __len__(self):
return len(self._values)

def add(self, value):
self._values.add(value)

def discard(self, value):
self._values.discard(value)

impl = SetUsingInstanceFromIterable([1, 2, 3], 'test')

actual = impl - {1}
self.assertIsInstance(actual, SetUsingInstanceFromIterable)
self.assertEqual('from_iterable', actual.created_by)
self.assertEqual({2, 3}, actual)

actual = impl | {4}
self.assertIsInstance(actual, SetUsingInstanceFromIterable)
self.assertEqual('from_iterable', actual.created_by)
self.assertEqual({1, 2, 3, 4}, actual)

actual = impl & {2}
self.assertIsInstance(actual, SetUsingInstanceFromIterable)
self.assertEqual('from_iterable', actual.created_by)
self.assertEqual({2}, actual)

actual = impl ^ {3, 4}
self.assertIsInstance(actual, SetUsingInstanceFromIterable)
self.assertEqual('from_iterable', actual.created_by)
self.assertEqual({1, 2, 4}, actual)

# NOTE: ixor'ing with a list is important here: internally, __ixor__
# only calls _from_iterable if the other value isn't already a Set.
impl ^= [3, 4]
self.assertIsInstance(impl, SetUsingInstanceFromIterable)
self.assertEqual('test', impl.created_by)
self.assertEqual({1, 2, 4}, impl)

def test_Set_interoperability_with_real_sets(self):
# Issue: 8743
class ListSet(Set):
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