Sounds good to me.
On Sun, Nov 12, 2017 at 7:17 AM, Serhiy Storchaka
Initially generator expressions always had to be written inside parentheses, as documented in PEP 289 [1]. The additional parenthesis could be omitted on calls with only one argument, because in this case the generator expression already is written inside parentheses. You could write just `list(x for x in [1])` instead of `list((x for x in [1]))`. The following code was an error:
list(x for x in [1], *[]) File "<stdin>", line 1 SyntaxError: invalid syntax list(x for x in [1],) File "<stdin>", line 1 SyntaxError: invalid syntax
You needed to add explicit parenthesis in these cases:
list((x for x in [1]), *[]) [1] list((x for x in [1]),) [1]
But in Python 2.5 the following examples were accepted:
list(x for x in [1], *[]) [1] list(x for x in [1], *{}) [1] list(x for x in [1],) [1]
However I haven't found anything about this change in the "What's New In Python 2.5" document [2].
The former two cases were found to be a mistake and it was fixed in Python 3.5.
list(x for x in [1], *[]) File "<stdin>", line 1 SyntaxError: Generator expression must be parenthesized if not sole argument list(x for x in [1], *{}) File "<stdin>", line 1 SyntaxError: Generator expression must be parenthesized if not sole argument
But `list(x for x in [1],)` still is accepted. I think it would be better it this raises a SyntaxError.
1. This syntax is ambiguous, because at first look it is not clear whether it is equivalent to `list((x for x in [1]),)` or to `list(x for x in ([1],))`.
2. It is bad from the aesthetic point of view, because this is the only case when the generator expression has not written inside parentheses. I believe that allowing to omit parenthesis in a call with a single generator expression argument was caused by aesthetic reasons.
3. I believe the trailing comma in function call was allowed because this simplified adding, removing and commenting out arguments.
func(first_argument, second_argument, #third_argument, )
You shouldn't touch other lines by adding or removing a comma when add or remove arguments. But this reason is not applicable to the case of `list((x for x in [1]),)`, because the generator expression without parenthesis should be the only argument. Therefore there is no reasons to allow this syntax.
4. 2to3 didn't supported this syntax for recent times [4]. Finally it was changed, but I think that it would be better to disallow this syntax for reasons mentioned above.
[1] https://www.python.org/dev/peps/pep-0289/ [2] https://docs.python.org/2.5/whatsnew/whatsnew25.html [3] https://docs.python.org/3.5/whatsnew/3.5.html#changes-in-pyt hon-behavior [4] https://bugs.python.org/issue27494
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