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