[Python-Dev] Disallow ambiguous syntax f(x for x in [1],)

[Guido van Rossum]

Sounds good to me.

On Sun, Nov 12, 2017 at 7:17 AM, Serhiy Storchaka <storchaka at gmail.com>
wrote:

> 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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-- 
--Guido van Rossum (python.org/~guido)
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