Serialization

Obviously OrderedDict would benefit (both __init__() and update()) from ordered kwargs. However, the benefit also extends to serialization APIs [2]:

In the context of serialisation, one key lesson we have learned is
that arbitrary ordering is a problem when you want to minimise
spurious diffs, and sorting isn't a simple solution.

Tools like doctest don't tolerate spurious diffs at all, but are
often amenable to a sorting based answer.

The cases where it would be highly desirable to be able use keyword
arguments to control the order of display of a collection of key
value pairs are ones like:

* printing out key:value pairs in CLI output
* mapping semantic names to column order in a CSV
* serialising attributes and elements in particular orders in XML
* serialising map keys in particular orders in human readable formats
  like JSON and YAML (particularly when they're going to be placed
  under source control)

Debugging

In the words of Raymond Hettinger [7]:

It makes it easier to debug if the arguments show-up in the order
they were created.  AFAICT, no purpose is served by scrambling them.

Other Use Cases

• Mock objects. [8]
• Controlling object presentation.
• Alternate namedtuple() where defaults can be specified.
• Specifying argument priority by order.

Performance

As already noted, the idea of ordered keyword arguments has come up on a number of occasions. Each time it has been met with the same response, namely that preserving keyword arg order would have a sufficiently adverse effect on function call performance that it’s not worth doing. However, Guido noted the following [9]:

Making **kwds ordered is still open, but requires careful design and
implementation to avoid slowing down function calls that don't benefit.

As will be noted below, there are ways to work around this at the expense of increased complication. Ultimately the simplest approach is the one that makes the most sense: pack collected key word arguments into an OrderedDict. However, without a C implementation of OrderedDict there isn’t much to discuss. That changed in Python 3.5. [10]

Note: in Python 3.6 dict is order-preserving. This virtually eliminates performance concerns.

Other Python Implementations

Another important issue to consider is that new features must be cognizant of the multiple Python implementations. At some point each of them would be expected to have implemented ordered kwargs. In this regard there doesn’t seem to be an issue with the idea. [11] An informal survey of the major Python implementations has indicated that this feature will not be a significant burden.

Relationship to **-unpacking syntax

The ** unpacking syntax in function calls has no special connection with this proposal. Keyword arguments provided by unpacking will be treated in exactly the same way as they are now: ones that match defined parameters are gather there and the remainder will be collected into the ordered kwargs (just like any other unmatched keyword argument).

Note that unpacking a mapping with undefined order, such as dict, will preserve its iteration order like normal. It’s just that the order will remain undefined. The ordered mapping into which the unpacked key-value pairs will then be packed will not be able to provide any alternate ordering. This should not be surprising.

There have been brief discussions of simply passing these mappings through to the functions kwargs without unpacking and repacking them, but that is both outside the scope of this proposal and probably a bad idea regardless. (There is a reason those discussions were brief.)

Relationship to inspect.Signature

Signature objects should need no changes. The kwargs parameter of inspect.BoundArguments (returned by Signature.bind() and Signature.bind_partial()) will change from a dict to an OrderedDict.

C-API

No changes.

Syntax

No syntax is added or changed by this proposal.

Backward-Compatibility

The following will change:

• iteration order of kwargs will now be consistent (except of course in the case described above)

Opt-out Decorator

This is identical to the current proposal with the exception that Python would also provide a decorator in functools that would cause collected keyword arguments to be packed into a normal dict instead of an OrderedDict.

Prognosis:

This would only be necessary if performance is determined to be significantly different in some uncommon cases or that there are other backward-compatibility concerns that cannot be resolved otherwise.

Opt-in Decorator

The status quo would be unchanged. Instead Python would provide a decorator in functools that would register or mark the decorated function as one that should get ordered keyword arguments. The performance overhead to check the function at call time would be marginal.

Prognosis:

The only real down-side is in the case of function wrappers factories (e.g. functools.partial and many decorators) that aim to perfectly preserve keyword arguments by using kwargs in the wrapper definition and kwargs unpacking in the call to the wrapped function. Each wrapper would have to be updated separately, though having functools.wraps() do this automaticallywould help.

__kworder__

The order of keyword arguments would be stored separately in a list at call time. The list would be bound to __kworder__ in the function locals.

Prognosis:

This likewise complicates the wrapper case.

Compact dict with faster iteration

Raymond Hettinger has introduced the idea of a dict implementation that would result in preserving insertion order on dicts (until the first deletion). This would be a perfect fit for kwargs. [5]

Prognosis:

The idea is still uncertain in both viability and timeframe.

Note that Python 3.6 now has this dict implementation.

***kwargs

This would add a new form to a function’s signature as a mutually exclusive parallel to **kwargs. The new syntax, ***kwargs (note that there are three asterisks), would indicate that kwargs should preserve the order of keyword arguments.

Prognosis:

New syntax is only added to Python under the most dire circumstances. With other available solutions, new syntax is not justifiable. Furthermore, like all opt-in solutions, the new syntax would complicate the pass-through case.

annotations

This is a variation on the decorator approach. Instead of using a decorator to mark the function, you would use a function annotation on **kwargs.

Prognosis:

In addition to the pass-through complication, annotations have been actively discouraged in Python core development. Use of annotations to opt-in to order preservation runs the risk of interfering with other application-level use of annotations.

dict.__order__

dict objects would have a new attribute, __order__ that would default to None and that in the kwargs case the interpreter would use in the same way as described above for __kworder__.

Prognosis:

It would mean zero impact on kwargs performance but the change would be pretty intrusive (Python uses dict a lot). Also, for the wrapper case the interpreter would have to be careful to preserve __order__.

KWArgsDict.__order__

This is the same as the dict.__order__ idea, but kwargs would be an instance of a new minimal dict subclass that provides the __order__ attribute. dict would instead be unchanged.

Prognosis:

Simply switching to OrderedDict is a less complicated and more intuitive change.