Re: Allowing zero-dimensional subscripts

Steve Holden wrote:
Hey, I have an idea, why don't we look at the language reference manual
instead of imagining how we think it might work!

I don't know. Sounds risky.


In section 3.2 we find:


"""
Tuples
The items of a tuple are arbitrary Python objects. Tuples of two or more
items are formed by comma-separated lists of expressions. A tuple of one
item (a `singleton') can be formed by affixing a comma to an expression
(an expression by itself does not create a tuple, since parentheses must
be usable for grouping of expressions). An empty tuple can be formed by
an empty pair of parentheses.
"""

So it seems that your speculation is false. Section 2.6 specifically
defines "[" and "]" as delimiters. Section 5.3.2 defines a subscription
(a term I've not really grown to love, but what the heck) as

subscription ::= primary "[" expression_list "]"

and section 5.12, which defines expression_list, explicitly says

"""An expression list containing at least one comma yields a tuple.""".

So it would appear that while your change might be very convenient to
allow you to refer to scalar values as zero-dimensional arrays, it
doesn't really fit into Python's conceptual framework. Sorry.

Yes, that would appear to be so. You would have a point... if the
documentation were correct. Only it's not.

According to the reference manual, the rule for an expression_list is:

expression_list ::= expression ( "," expression )* [","]

But take the following legal Python subscripted array:

a[1:2,...,3:4]

Is "1:2" an expression? How about "..."? When I enter 1:2 at the
Python prompt, I get a syntax error. The fact is, the documentation
here is either wrong or simplified or both. (I don't think it's a big
deal, actually: the real grammar has lots of complexity to handle
tricky cases that would needlessly complicate the reference manual for
a human reader.) So let's look at an excerpt the actual Python grammar
(from 2.4.3). You'll be happy to know subscription isn't used. :)

trailer: '(' [arglist] ')' | '[' subscriptlist ']' | '.' NAME
subscriptlist: subscript (',' subscript)* [',']
sliceop: ':' [test]
subscript: '.' '.' '.' | test | [test] ':' [test] [sliceop]
testlist: test (',' test)* [',']

Clearly, the grammar rule used for list subscript is different from the
one used for list of expressions (for some reason, what an ordinary
person would call an expression is called a "test" in the grammar,
whereas "expr" is a non-short-circuiting expression).

So there's a regular way to create non-empty tuples, and a subscript
way.

And there's a regular way to create an empty tuple... but not a
subscript way.

So I'd say this change fits the conceptual framework of the tuple quite
well; in fact, it makes subscript tuples more parallel to their regular
counterparts.


One further point: if you really do conceptualize scalars as
zero-dimensional arrays, where is the value conceptually stored?

Think of it this way: an array with n-dimensions of length 3 would have
3**n total entries. How many entries would a 0-dimensional array have?
3**0 == 1.

Numeric has had zero-dimensional arrays for a long time, and has had no
problem storing them. Think of the rule for accessing an element of an
array: it's a base pointer + sum (indices*stride) for all indices. Now
generalize it down to zero: there are no indices, so the scalar is
stored at the base pointer.


Carl Banks

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Relevant Pages

  • Re: Allowing zero-dimensional subscripts
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