frozenset could be "updated" by |= operator, is this intended?

[DanielYang59]

Bug report

Bug description:

The |= operator behaves unexpectedly when used with frozenset, where it creates a new instance. This can be misleading because it effectively updates it. I would expect this to trigger a TypeError?

fs = frozenset([1, 2, 3])

print(id(fs))  # 4379466816

fs |= {4, 5}

print(fs)  # >>> frozenset({1, 2, 3, 4, 5})

print(id(fs))  # ID changed to 4379462560

CPython versions tested on:

3.9, 3.10, 3.11, 3.12, 3.13

Operating systems tested on:

macOS

[nineteendo]

No, only the reference is updated:

>>> set1 = set2 = frozenset([1, 2, 3])
>>> set2 |= {4, 5} # equivalent to set2 = set2 | {4, 5}
>>> set1
frozenset({1, 2, 3})
>>> set2
frozenset({1, 2, 3, 4, 5})

[skirpichev]

I don't anything wrong in this, consider:

>>> i = 1234
>>> id(i)
140639472689744
>>> i += 1
>>> i
1235
>>> id(i)
140639472686640

On another hand, docs says: "The following table lists operations available for set that do not apply to immutable instances of frozenset:
update(*others)
set |= other | ...

Update the set, adding elements from all others."

Seems to be a docs issue.

[ZeroIntensity]

If you're interested in mutating immutable objects, please look into footguns such as ctypes 😄

I don't see anything that looks like a bug here.

[nineteendo]

https://docs.python.org/3/reference/datamodel.html#object.__ior__:

For instance, if x is an instance of a class with an __iadd__ method, x += y is equivalent to x = x.__iadd__(y) . If __iadd__ does not exist, or if x.__iadd__(y) returns NotImplemented, x.__add__(y) and y.__radd__(x) are considered, as with the evaluation of x + y.

>>> frozenset().__or__
<method-wrapper '__or__' of frozenset object at 0x7f84dbd25900>
>>> frozenset().__ior__
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
AttributeError: 'frozenset' object has no attribute '__ior__'

[DanielYang59]

Thanks you guys are crazy responsive :)

On another hand, docs says: "The following table lists operations available for set that do not apply to immutable instances of frozenset:

Yes I'm not expecting these operations to work for frozenset (I expect a TypeError), but for some reason they do?

[nineteendo]

Read my comment above, the same applies to __ior__().

[ZeroIntensity]

Maybe we could have decided it could raise a TypeError 20 years ago. Now, changing that would break untold amounts of code. Sorry :(

[DanielYang59]

Read my comment above, the same applies to ior().

Thanks that's really helpful. I thought |= would internally call __ior__ exactly and it turns out it falls back to x = x.__or__(y). I really appreciate this :)

Maybe we could have decided it could raise a TypeError 20 years ago. Now, changing that would break untold amounts of code. Sorry :(

Understandable :)

[DanielYang59]

Again you guys are amazingly responsive and helpful, really appreciate that!

[DanielYang59]

On another hand, docs says: "The following table lists operations available for set that do not apply to immutable instances of frozenset:

Putting the breaking implementation change aside, can I ask for a documentation clarification? As the docs explicitly says "do not apply to immutable instances of frozenset" while they do work?

[ZeroIntensity]

They work, but those methods don't behave as documented on frozenset as they do set.