[Python-ideas] PEP pre-draft: Support for indexing with keyword arguments

[Stefano Borini]

On Wed, Jul 02, 2014 at 12:36:48AM +0200, Stefano Borini wrote:
> https://github.com/stefanoborini/pep-keyword/blob/master/PEP-XXX.txt

I just added a new strategy. This one cuts the problem down.

Strategy 4: Strict dictionary
-----------------------------

This strategy accepts that __getitem__ is special in accepting only one object,
and the nature of that object must be non-ambiguous in its specification of the
axes: it can be either by order, or by name. As a result of this assumption,
in presence of keyword arguments, the passed entity is a dictionary and all
labels must be specified.

    C0. a[1]; a[1,2]      -> idx = 1; idx=(1, 2)
    C1. a[Z=3]            -> idx = {"Z": 3}
    C2. a[Z=3, R=4]       -> idx = {"Z"=3, "R"=4}
    C3. a[1, Z=3]         -> raise SyntaxError
    C4. a[1, Z=3, R=4]    -> raise SyntaxError
    C5. a[1, 2, Z=3]      -> raise SyntaxError
    C6. a[1, 2, Z=3, R=4] -> raise SyntaxError
    C7. a[1, Z=3, 2, R=4] -> raise SyntaxError


Pros:
    - strong conceptual similarity between the tuple case and the dictionary case.
      In the first case, we are specifying a tuple, so we are naturally defining
      a plain set of values separated by commas. In the second, we are specifying a
      dictionary, so we are specifying a homogeneous set of key/value pairs, as
      in dict(Z=3, R=4)
    - simple and easy to parse on the __getitem__ side: if it gets a tuple, 
      determine the axes using positioning. If it gets a dictionary, use 
      the keywords.
    - C interface does not need changes.

Cons:
    - degeneracy of a[{"Z": 3, "R": 4}] with a[Z=3, R=4], but the same degeneracy exists
      for a[(2,3)] and a[2,3].
    - very strict.
    - destroys the use case a[1, 2, default=5]


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