Christopher wrote: Why not allow slice syntax as an expression everywhere?
In reply, Todd wrote: That is a very different discussion, and not directly
related to keyword indexes. Would it be possible to start a new email
thread to discuss it?
I think they are closely related matters, at least in terms of
implementation. For details see rest of this message. I hope this helps our
understanding, even if it shows difficulties lying ahead.
My non-expert understanding is that if
>>> d[a=1:2:3]
is allowed by making a minimal change to Python's abstract grammar, then
>>> f(a=1:2:3)
will also be allowed. (Extra work would be required to forbid it.)
It is also my non-expert understanding that
>>> {0:1:2:3:4:5}
would then be equivalent to
>>> {slice(0, 1, 2): slice(3, 4, 5)}
and further that
>>> { :: :: : }
would become valid syntax!
My non-expert understanding is based on
https://docs.python.org/3/library/ast.html#abstract-grammar
To me it seems that in for example
>>> d[::, ::]
the AST is constrained by
slice = Slice(expr? lower, expr? upper, expr? step)
| ExtSlice(slice* dims)
| Index(expr value)
while in
>>> f(x=SOMETHING)
>>> f[x=SOMETHING]
the SOMETHING is an expr, and the AST is constrained by
expr = BoolOp(boolop op, expr* values)
| NamedExpr(expr target, expr value)
| BinOp(expr left, operator op, expr right)
| UnaryOp(unaryop op, expr operand)
| Lambda(arguments args, expr body)
| IfExp(expr test, expr body, expr orelse)
| Dict(expr* keys, expr* values)
...
| List(expr* elts, expr_context ctx)
| Tuple(expr* elts, expr_context ctx
If this is correct then adding Slice to the choices for expr would extend
the AST to allow slices in keyword indices. And then the rest follows.
--
Jonathan
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