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?
Specification. 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.
Kwargs allow you to pass keyword arguments to a function. They are used when you are not sure of the number of keyword arguments that will be passed in the function. Kwargs can be used for unpacking dictionary key, value pairs. This is done using the double asterisk notation ( ** ).
It is often used if you want to pass lots of arguments to another function where you don't necessarily know the options. For instance, specialplot(a,**kwargs) can pass plot options in kwargs to a generic plot function that accepts those options as named parameters.
Use the Python **kwargs parameter to allow the function to accept a variable number of keyword arguments. Inside the function, the kwargs argument is a dictionary that contains all keyword arguments as its name-value pairs. Precede double stars ( ** ) to a dictionary argument to pass it to **kwargs parameter.
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 aslist(od1.items())==list(od2.items())
. Equality tests betweenOrderedDict
objects and otherMapping
objects are order-insensitive like regular dictionaries. This allowsOrderedDict
objects to be substituted anywhere a regular dictionary is used.
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 dict
s 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.
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.
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