[Python-ideas] Re: Notation for subscripts.
On 2021-08-13 at 23:18:29 +1100,
Matsuoka Takuo wrote:
> Given a subscriptable object s, the intended rule for the notation for
> getting an item of s seems that, for any expression {e}, such as
> "x, ",
> s[{e}]
> (i.e., s[x, ] if {e} is "x, ") means the same as
> s[({e})]
If e is an expression, then s[e] means s[e].
Sometimes, e happens to be a tuple, but Python doesn't create a tuple
just to call __getitem__. The following expression:
x,
is a tuple all by itself. Also:
(x,)
is that same tuple.
> (i.e., s[(x, )] in the considered case), namely, should be evaluated
> as s.__getitem__(({e})) (or s.__class_getitem__(({e})) when that
> applies). If this is the rule, then it looks simple and hence
> friendly to the user. However, there are at least two exceptions:
>
> (1) The case where {e} is the empty expression "":
There is no such thing as an "empty expression." "" is a string
containing no characters.
> The expression
> s[]
> raises SyntaxError ...
Because there is no expression (which is different from a hypothetical
"empty expression."
> ... instead of being evaluated in the same way as
> s[()] is.
() is a tuple with no elements, just as [] is a list with no elements,
and "" is a string with no elements.
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[Python-ideas] Notation for subscripts.
Dear Developers,
Given a subscriptable object s, the intended rule for the notation for
getting an item of s seems that, for any expression {e}, such as
"x, ",
s[{e}]
(i.e., s[x, ] if {e} is "x, ") means the same as
s[({e})]
(i.e., s[(x, )] in the considered case), namely, should be evaluated
as s.__getitem__(({e})) (or s.__class_getitem__(({e})) when that
applies). If this is the rule, then it looks simple and hence
friendly to the user. However, there are at least two exceptions:
(1) The case where {e} is the empty expression "":
The expression
s[]
raises SyntaxError instead of being evaluated in the same way as
s[()] is.
(2) The case where {e} contains "*" for unpacking:
An expression containing the unpacking notation, such as
s[*iterable, ]
raises SyntaxError instead of being evaluated in the same way as
s[(*iterable, )] in this example, is.
Are these (and other if any) exceptions justified? If not, I propose
having the described rule to have full effect if that would simplify
the syntax. This would affect currently working codes which rely on
SyntaxError raised in either of the described ways (through eval, exec
or import??). I wonder if reliance on SyntaxError in these cases
should be supported in all future versions of Python.
Best regards,
Takuo Matsuoka
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