In 3.8: >>> {1: 2, 3: 4}.keys() & {1, 2} {1} In master: >>> {1: 2, 3: 4}.keys() & iter([1, 2]) set() The behavior was changed in issue27575.

 New changeset c38e725d17537b20ff090b1b5ec7db1820ff9b63 by Serhiy Storchaka (Dong-hee Na) in branch 'master': bpo-38210: Fix intersection operation with dict view and iterator. (GH-16602) https://github.com/python/cpython/commit/c38e725d17537b20ff090b1b5ec7db1820ff9b63 

Thank you Dong-hee for your fix!

Please add a test for this regression.

It was added. Do you mean any special?

 New changeset d97f1ce6dba6c4aa5614adc06ad2e0948709845c by Victor Stinner in branch 'master': bpo-38210: Fix compiler warning in dictobject.c (GH-16611) https://github.com/python/cpython/commit/d97f1ce6dba6c4aa5614adc06ad2e0948709845c 

This caused an unintentional behavior change in the following code: >>> {1: 2}.items() & {1: {2: 3}}.items() set() Before this change, Python 3.6 - 3.8 behaved like this instead: >>> {1: 2}.items() & {1: {2: 3}}.items() Traceback (most recent call last): File "<stdin>", line 1, in <module> TypeError: unhashable type: 'dict' Interestingly, this doesn't seem to have a negative effect on correctness as the silently omitted unhashable (k, v) pair is only omitted if it's different between the two dictionaries: >>> {1: {2: 4}}.items() & {1: {2: 3}}.items() set() >>> {2: 1, 1: {2: 4}}.items() & {2: 1, 1: {2: 3}}.items() {(2, 1)} If it's the same, we still get an error in Python 3.9: >>> {1: {2: 3}}.items() & {1: {2: 3}}.items() Traceback (most recent call last): File "<stdin>", line 1, in <module> TypeError: unhashable type: 'dict' >>> {2: 1, 1: {2: 3}}.items() & {2: 1, 1: {2: 3}}.items() Traceback (most recent call last): File "<stdin>", line 1, in <module> TypeError: unhashable type: 'dict'

>Interestingly, this doesn't seem to have a negative effect on correctness as the silently omitted unhashable I think so too. The error actually raises when adding the object into the set. https://github.com/python/cpython/blob/818628c2da99ba0376313971816d472c65c9a9fc/Objects/dictobject.c#L4384 Since the target object to be added is dynamically generated, I think that the issue does not need to be fixed. Otherwise, we have to check that all objects are addable to `set` object before executing this operation but it looks harmful to performance.