*() Invalid Syntax: iterable unpacking of empty tuple

[spralja]

BPO	46865
Nosy	@serhiy-storchaka, @spralja

^{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 2022-02-26.09:07:18.358>
created_at = <Date 2022-02-26.09:04:32.102>
labels = ['invalid', '3.9']
title = '*() Invalid Syntax: iterable unpacking of empty tuple'
updated_at = <Date 2022-02-26.10:09:17.306>
user = 'https://github.com/spralja'

bugs.python.org fields:

activity = <Date 2022-02-26.10:09:17.306>
actor = 'serhiy.storchaka'
assignee = 'none'
closed = True
closed_date = <Date 2022-02-26.09:07:18.358>
closer = 'serhiy.storchaka'
components = []
creation = <Date 2022-02-26.09:04:32.102>
creator = 'spralja'
dependencies = []
files = []
hgrepos = []
issue_num = 46865
keywords = []
message_count = 4.0
messages = ['414085', '414086', '414087', '414088']
nosy_count = 2.0
nosy_names = ['serhiy.storchaka', 'spralja']
pr_nums = []
priority = 'normal'
resolution = 'not a bug'
stage = 'resolved'
status = 'closed'
superseder = None
type = None
url = 'https://bugs.python.org/issue46865'
versions = ['Python 3.9']

[spralja]

`
>>> def foo(num=1):
...     return num
...
>>> foo(*(bool,) is bool else *())
  File "<stdin>", line 1
    foo(*(bool,) is bool else *())
                         ^
SyntaxError: invalid syntax
>>> foo(*(bool,) if bool else *())
  File "<stdin>", line 1
    foo(*(bool,) if bool else *())
                              ^
SyntaxError: invalid syntax
>>> def foo(num=1):
...     return num
...
>>> stri = ''
>>> foo(*(stri,) if stri else *())
  File "<stdin>", line 1
    foo(*(stri,) if stri else *())
                              ^
SyntaxError: invalid syntax
>>> foo(*((stri,) if stri else ()))
1
>>>
`

Iterable unpacking of empty tuple seems to not work in one example but does in another.

[serhiy-storchaka]

It is an invalid syntax. Write foo(*((stri,) if stri else ())).

[spralja]

I understand that it's invalid synatax, but not why.

On Sat, 26 Feb 2022 at 10:07, Serhiy Storchaka <report@bugs.python.org>
wrote:

Serhiy Storchaka <storchaka+cpython@gmail.com> added the comment:

It is an invalid syntax. Write foo(*((stri,) if stri else ())).

----------
nosy: +serhiy.storchaka
resolution: -> not a bug
stage: -> resolved
status: open -> closed

---

Python tracker <report@bugs.python.org>
<https://bugs.python.org/issue46865\>

---

[serhiy-storchaka]

See the grammar. https://docs.python.org/3/reference/expressions.html#conditional-expressions

conditional_expression ::= or_test ["if" or_test "else" expression]
expression ::= conditional_expression | lambda_expr

*() is not an expression. It is either a starred_item (which can be a part of starred_list or a starred_expression), or a positional_item or a starred_and_keywords which are parts of an argument_list. None of them are expressions.

foo(*(stri,) if stri else x) is interpreted as foo(*((stri,) if stri else x)) if x is an expression. But *() is not an expression, thus a syntax error.