> On May 3, 2020, at 6:19 PM, Steven D'Aprano <st...@pearwood.info> wrote:
>
>>> `frozenset` and `set` make a counterexample:
>>>
>>>>>> frozenset({1}) == {1}
>>> True
>>>
>>
>> Nice catch! That's really interesting. Is there reasoning behind
>> `frozenset({1}) == {1}` but `[1] != (1,)`, or is it just an accident of
>> history?
>
> Conceptually, sets are sets, whether they are mutable or frozen.
Right. This isn't an accident. It is by design.
Also, some numeric types are specifically designed for cross-type comparison:
>>> int(3) == float(3) == complex(3, 0)
True
And in Python 2, by design, str and unicode were comparable:
>>> u'abc' == 'abc'
True
But the general rule is that objects aren't cross-type comparable by default.
We have to specifically enable that behavior when we think it universally makes
sense. The modern trend is to avoid cross-type comparability, enumerates and
data classes for example:
>>> Furniture = Enum('Furniture', ('table', 'chair', 'couch'))
>>> HTML = Enum('HTML', ('dl', 'ol', 'ul', 'table'))
>>> Furniture.table == HTML.table
False
>>> A = make_dataclass('A', 'x')
>>> B = make_dataclass('B', 'x')
>>> A(10) == B(10)
False
Bytes and str are not comparable in Python 3:
>>> b'abc' == 'abc'
False
>> Isn't a tuple essentially just a frozenlist? I know the intended
>> semantics of tuples and lists tend to be different, but I'm not sure that's
>> relevant.
In terms of API, it might look that way. But in terms of use cases, they are
less alike: lists-are-looping, tuples-are-for-nonhomongenous-fields. List are
like database tables; tuples are like records in the database. Lists are like
C arrays; tuples are like structs.
On the balance, I think more harm than good would result from making sequence
equality not depend on type. Also when needed, it isn't difficult to be
explicit that you're converting to a common type to focus on contents:
>>> s = bytes([10, 20, 30])
>>> t = (10, 20, 30)
>>> list(s) == list(t)
When you think about it, it makes sense that a user gets to choose whether
equality is determined by contents or by contents and type. For some drinkers,
a can of beer is equal to a bottle of bear; for some drinkers, they aren't
equal at all ;-)
Lastly, when it comes to containers. They each get to make their own rules
about what is equal. Dicts compare on contents regardless of order, but
OrderedDict requires that the order matches.
Raymond
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