"Data blocks" syntax specification draft

[Ian Kelly]

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].

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

py> def foo():
...   return 1, 2
...
py> foo()
(1, 2)

py> def foo():
...   return 1, 2, *range(3, 5)
  File "<stdin>", line 2
    return 1, 2, *range(3, 5)
                 ^
SyntaxError: invalid syntax

py> def foo():
...   yield 1, 2
...
py> list(foo())
[(1, 2)]
py> def foo():
...   yield 1, 2, *range(3, 5)
  File "<stdin>", line 2
    yield 1, 2, *range(3, 5)
                ^
SyntaxError: invalid syntax

py> for x in 1, 2: print(x)
...
1
2
py> for x in 1, 2, *range(3, 5): print(x)
  File "<stdin>", line 1
    for x in 1, 2, *range(3, 5): print(x)
                   ^
SyntaxError: invalid syntax