Always require parentheses for genexps in base class lists

[ncoghlan]

BPO	32023
Nosy	@ncoghlan, @serhiy-storchaka
PRs	bpo-32023: Disallow genexprs without parenthesis in class definitions. #4400
Dependencies	bpo-32012: Disallow ambiguous syntax f(x for x in [1],)

^{Note: these values reflect the state of the issue at the time it was migrated and might not reflect the current state.}

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GitHub fields:

assignee = None
closed_at = <Date 2017-11-15.15:54:47.629>
created_at = <Date 2017-11-14.10:25:03.999>
labels = ['interpreter-core', 'type-bug', '3.7']
title = 'Always require parentheses for genexps in base class lists'
updated_at = <Date 2017-11-15.15:54:47.628>
user = 'https://github.com/ncoghlan'

bugs.python.org fields:

activity = <Date 2017-11-15.15:54:47.628>
actor = 'serhiy.storchaka'
assignee = 'none'
closed = True
closed_date = <Date 2017-11-15.15:54:47.629>
closer = 'serhiy.storchaka'
components = ['Interpreter Core']
creation = <Date 2017-11-14.10:25:03.999>
creator = 'ncoghlan'
dependencies = ['32012']
files = []
hgrepos = []
issue_num = 32023
keywords = ['patch']
message_count = 2.0
messages = ['306197', '306275']
nosy_count = 2.0
nosy_names = ['ncoghlan', 'serhiy.storchaka']
pr_nums = ['4400']
priority = 'low'
resolution = 'fixed'
stage = 'resolved'
status = 'closed'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue32023'
versions = ['Python 3.7']

[ncoghlan]

The compiler currently allows parentheses to be omitted if a generator expression is the sole entry in a base class list:

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

The language reference states that the parentheses around a generator expression are only optional for "calls with only one argument": https://docs.python.org/3/reference/expressions.html#generator-expressions

A base class list is not a call, so this should be treated as a syntax error, rather than being handled as equivalent to class C((x for x in [])): pass

[serhiy-storchaka]

New changeset ddbce13 by Serhiy Storchaka in branch 'master':
bpo-32023: Disallow genexprs without parenthesis in class definitions. (bpo-4400)
ddbce13