"Data blocks" syntax specification draft

[Ian Kelly]

On Tue, May 22, 2018 at 9:34 AM, Chris Angelico <rosuav at gmail.com> wrote:
> On Wed, May 23, 2018 at 1:22 AM, Ian Kelly <ian.g.kelly at gmail.com> wrote:
>> On Tue, May 22, 2018 at 8:25 AM, Chris Angelico <rosuav at gmail.com> wrote:
>>> On Tue, May 22, 2018 at 8:25 PM, bartc <bc at freeuk.com> wrote:
>>>> Note that Python tuples don't always need a start symbol:
>>>>
>>>>    a = 10,20,30
>>>>
>>>> assigns a tuple to a.
>>>
>>> The tuple has nothing to do with the parentheses, except for the
>>> special case of the empty tuple. It's the comma.
>>
>> Although, if the rule were really as simple as "commas make tuples",
>> then this would be a list containing a tuple: [1, 2, 3].
>
> In an arbitrary expression, a comma between two expressions creates a
> tuple. In other contexts, the comma has other meanings, which take
> precedence:
>
> * Separating a function's arguments (both at definition and call)
> * Enumerating import targets and global/nonlocal names
> * Separating an assertion from its message
> * Listing multiple context managers
> * And probably some that I've forgotten.
>
> In those contexts, you can override the normal interpretation and
> force the tuple by using parentheses, preventing it from being parsed
> as something else, and making it instead a single expression:
>
> print((1, 2)) # prints a tuple
> print(1, 2) # prints two items
>
> The comma is what makes the tuple, though, not the parentheses. The
> parentheses merely prevent this from being something else.

In other words, the rule is not really as simple as "commas make
tuples". I stand by what I wrote.

>> Curiously, parentheses are also sometimes required for iterable
>> unpacking. For example:
>>
>> py> 1, 2, *range(3,5)
>> (1, 2, 3, 4)
>> py> d = {}
>> py> d[1, 2] = 42
>> py> d[1, 2, *range(3,5)] = 43
>>   File "<stdin>", line 1
>>     d[1, 2, *range(3,5)] = 43
>>             ^
>> SyntaxError: invalid syntax
>
> I'm not sure what you mean about the parentheses here. AIUI iterable
> unpacking simply isn't supported inside subscripting. If that's an
> actual problem anywhere, I'm sure it could be added :)

Of course it's supported:

py> d = {}
py> d[(1, 2, *range(3, 5))] = 43
py> d
{(1, 2, 3, 4): 43}

Works just fine. But take out the parentheses and you get the SyntaxError.