"Data blocks" syntax specification draft

[Steven D'Aprano]

On Wed, 23 May 2018 11:10:33 +0100, bartc wrote:

[...]
>> You haven't done enough testing. All you have done is found that "round
>> brackets give a tuple, other brackets don't". But you need to test what
>> happens if you take away the brackets to be sure that it is the round
>> brackets which create the tuple:
>> 
>>      a = 10, 20, 30  # take away the ()
>> 
>> You still get a tuple. Taking away the [] and {} also give tuples.
> 
> If ... is a comma-separated sequence of (2 or more) expressions, then:
> 
>   Enclosed with:             It yields:
> 
>   {...} brackets             Set (or dict with key:value items)
>   [...] brackets             List
>   (...) brackets             Tuple
>    ...  (no) brackets        Tuple

Indeed.

> Each ... contains commas. But it is what surrounds ..., or that doesn't
> surround ..., that determines what the construction yields. The commas
> are not specific to tuples.

Nobody said that they were. You are arguing against a strawman. The 
brackets are not part of tuple syntax. They are sometimes needed for 
grouping, when some other grammatical construct would otherwise take 
precedence. That is all.


>> What happens if you add extra brackets?
>> 
>>      a = ((10, 20, 30))  # tuple
>>      b = ([10, 20, 30])  # list
>>      c = ({10, 20, 30})  # set
> 
> 0 items within the list:
> 
> ()    Empty tuple
> []    Empty list
> {}    Empty dict

Aye ... as we've acknowledged numerous times now, the empty tuple *is* a 
genuine special case, one which *does* rely on an empty pair of round 
brackets.



> 1 item within the list
> 
> (x)   Yields x

Exactly! There is no comma, therefore, there is no tuple.

> [x]   List of one item
> {x}   Set of one item

Neither of these require commas, since you don't need a separator to 
separate a single item. The square brackets *do* specify that the item is 
a list; the curly brackets *do* specify a dict or set. The round brackets 
DO NOT specify a tuple (except in the empty tuple case).

No comma, no tuple, no matter how emphatically you insert round brackets 
around the item.


> Because () is used for normal bracketing of expression terms, it
> requires this special form to denote a tuple:
> 
> (x,)  Tuple of one item

Incorrect. Yet again, you have failed to do enough testing. No special 
form is required. Only a comma:

py> x = 1,
py> type(x)
<class 'tuple'>

It isn't enough to test examples which confirm a hypothesis. You need to 
test examples which also refute it, and see if your hypothesis survives 
the challenge.

Certainly commas are used as separators. They're used as separators in 
many different contexts: import statements, function calls, function 
declarations, class declarations, except statements, list displays, etc. 
We don't dispute that or deny it.

But in the context of "tuple displays" (the official terminology for 
tuple so-called "literal" syntax), the brackets are not part of the tuple 
display. It is the commas that tell the interpreter that the result is a 
tuple:

https://docs.python.org/3/reference/expressions.html#expression-lists

Because commas are used as separators in so many places, there is often a 
clash between one part of the grammar that uses commas and their use in 
tuples, requiring us to disambiguate the syntax with brackets.

https://docs.python.org/3/reference/expressions.html#parenthesized-forms


> which can ALSO be used to form one element lists, sets and dicts.

But *unlike* one-element lists, sets and dicts, where the comma is 
optional, it is *required* for tuples.


[...]
> Suppose you were parsing Python source, had just processed a term of an
> expression, and the next token was a comma. What came before the term
> may have been (, {, [ or nothing. What comes next, would you call:
[...]

I don't even understand what point you think you are making here. But it 
really doesn't matter. Ultimately, the grammar and documentation trumps 
any hypotheses we might have about what the language is doing, and they 
are absolutely clear about what is going on:


https://docs.python.org/3/reference/expressions.html#expression-lists
https://docs.python.org/3/reference/expressions.html#parenthesized-forms


-- 
Steve