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

Serhiy Storchaka storchaka at gmail.com
Sun Nov 12 10:17:51 EST 2017

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(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.

  1. 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-python-behavior [4] https://bugs.python.org/issue27494

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