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

Guido van Rossum guido at python.org
Sun Nov 12 11:57:46 EST 2017

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 "", line 1 SyntaxError: invalid syntax >>> list(x for x in [1],) File "", 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 "", line 1 SyntaxError: Generator expression must be parenthesized if not sole argument >>> list(x for x in [1], *{}) File "", 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(firstargument, secondargument, #thirdargument, ) 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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