On 2019-07-23 21:59, Kristian Klette wrote:
Hi!
During the sprints after EuroPython, I made an attempt at adding support for
comparing the results from `.values()` of two dicts.
Currently the following works as expected:
```
d = {'a': 1234}
d.keys() == d.keys()
d.items() == d.items()
```
but `d.values() == d.values()` does not return the expected
results. It always returns `False`. The symmetry is a bit off.
In the bug trackers[0] and the Github PR[1], I was asked
to raise the issue on the python-dev mailing list to find
a consensus on what comparing `.values()` should do.
I'd argue that Python should compare the values as expected here,
or if we don't want to encourage that behaviour, maybe we should
consider raising an exception.
Returning just `False` seems a bit misleading.
What are your thoughts on the issue?
Best regards,
Kristian Klette
[0]: https://bugs.python.org/issue37585
[1]: https://github.com/python/cpython/pull/14737
Well, the keys can function as a set, and, in fact, can equal a set:
>>> {'a': 1234}.keys() == {'a'}
True
so the keys of 2 dicts can be compared efficiently.
The items of 2 dicts can also be compared efficiently because you still
have the keys, so you can check the key efficiently and then check the
value.
However, when comparing the values you have a problem: you have 2
collections of objects that might contain duplicates, might not be
hashable, and might not be sortable, so comparing them could be
inefficient, and you can't refer back to their keys like in the case of
comparing the items as above because the 2 dicts might have different
keys. Unless someone can come up with an efficient solution, I'd
probably go with raising an exception.
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