Why does the **kwargs mapping compare equal with a

问题:

According to PEP 468:

Starting in version 3.6 Python will preserve the order of keyword arguments as passed to a function. To accomplish this the collected kwargs will now be an ordered mapping. Note that this does not necessarily mean OrderedDict.

In that case, why does this ordered mapping fail to respect equality comparison with Python's canonical ordered mapping type, the collections.OrderedDict:

>>> from collections import OrderedDict
>>> data = OrderedDict(zip('xy', 'xy'))
>>> def foo(**kwargs):
...     return kwargs == data
... 
>>> foo(x='x', y='y')  # expected result: True
True
>>> foo(y='y', x='x')  # expected result: False
True

Although iteration order is now preserved, kwargs seems to be behaving just like a normal dict for the comparisons. Python has a C implemented ordered dict since 3.5, so it could conceivably have been used directly (or, if performance was still a concern, a faster implementation using a thin subclass of the 3.6 compact dict).

Why doesn't the ordered mapping received by a function respect ordering in equality comparisons?

回答1:

The answer to your first 'why' is because this feature is implemented by using a plain dict in CPython. As @Ryan's answer points out, this means that comparisons won't be order-sensitive.

The second 'why' here is why this doesn't use an OrderedDict.

Using an OrderedDict was the initial plan as stated in the first draft of PEP 486. The idea, as stated in this reply, was to collect some perf data to show the effect of plugging in the OrderedDict since this was a point of contention when the idea was floated around before. The author of the PEP even alluded to the order preserving dict being another option in the final reply on that thread.

After that, the conversation on the topic seems to have died down until Python 3.6 came along. When the new dict came, it had the nice side-effect of just implementing PEP 486 out of the box (as this Python-dev thread states). The specific message in that thread also states how the author wanted the term OrderedDict to be changed to Ordered Mapping. (This is also when a new commit on PEP 468, after the initial one, was made)

As far as I can tell, this rewording was done in order to allow other implementations to provide this feature as they see fit. CPython and PyPy already had a dict that easily implemented PEP 468, other implementations might opt for an OrderedDict, others could go for another form of an ordered mapping.

That does open the door for a problem, though. It does mean that, theoretically, in an implementation of Python 3.6 with an OrderedDict as the structure implementing this feature, the comparison would be order-sensitive while in others (CPython) it wouldn't. (In Python 3.7, all dicts are required to be insertion-ordered so this point is probably moot since all implementations would use it for **kwargs)

Though it does seem like an issue, it really isn't. As @user2357112 pointed out, there's no guarantee on ==. PEP 468 only guarantees order. As far as I can tell, == is basically implementation defined.

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In short, it compares equal in CPython because kwargs in CPython is a dict and it's a dict because after 3.6 the whole thing just worked.

回答2:

Regardless of what an “ordered mapping” means, as long as it’s not necessarily OrderedDict, OrderedDict’s == won’t take into account its order. Docs:

Equality tests between OrderedDict objects are order-sensitive and are implemented as list(od1.items())==list(od2.items()). Equality tests between OrderedDict objects and other Mapping objects are order-insensitive like regular dictionaries. This allows OrderedDict objects to be substituted anywhere a regular dictionary is used.

回答3:

"Ordered mapping" only means the mapping has to preserve order. It doesn't mean that order has to be part of the mapping's == relation.

The purpose of PEP 468 is just to preserve the ordering information. Having order be part of == would produce backward incompatibility without any real benefit to any of the use cases that motivated PEP 468. Using OrderedDict would also be more expensive (since OrderedDict still maintains its own separate linked list to track order, and it can't abandon that linked list without sacrificing big-O efficiency in popitem and move_to_end).

回答4:

Just to add, if you do want to make this check (without relying on an implementation detail (which even then, won't be in python 3.7)), just do

from collections import OrderedDict
>>> data = OrderedDict(zip('xy', 'xy'))
>>> def foo(**kwargs):
...     return OrderedDict(kwargs) == data

since this is guaranteed to be True.