bpo-32012: Disallow trailing comma after genexpr without parenthesis. by serhiy-storchaka · Pull Request #4382 · python/cpython

What's wrong with this one? While we've restricted the form that decorator expressions take to being a name (dotted or otherwise) plus an optional function call, we've never restricted the arguments passed to that function call.

At the generator expression level, it also still meets the "must be surrounded by parentheses rule".

The same as with

or

or

This looks like valid Python expression syntax, but the syntax of decorator expression is intentionally more limited.

In any case these syntax changes should be discussed and officially approved. Could you please open a topic on Python-Dev about omitting parenthesis in a generator expression in these two cases? I have arguments for these changes and against them, but I think you will formulate them better. I'll add my comments.

Despite the fact that currently the CPython parser allows such syntax, it is illegal, and allowing it officially is a changing of Python language specification.

This one seems a bit odd to me as well, as we don't disallow other expressions in base class lists at compile time just because we know they can't produce a valid base class:

>>> class C([]): pass
... 
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: list() takes at most 1 argument (3 given)
>>> class C({}): pass
... 
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: dict expected at most 1 arguments, got 3

You still can use

>>> class C((1 for x in [1])): pass
... 
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: cannot create 'generator' instances

Omitting parenthesis in a generator expression is a special case for improving readability when pass a sole generator expression argument. The syntax of class definition doesn't have such special case.

Python/ast.c:614:72: error: unknown type name ‘bool’; did you mean ‘_Bool’?
Python/ast.c:1548:53: error: ‘false’ undeclared (first use in this function); did you mean ‘fabsl’?
Python/ast.c:2371:60: error: ‘true’ undeclared (first use in this function); did you mean ‘time’?