Re: 0 + not 0

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Date	Sat, 11 Jul 2015 17:20:58 +0100
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Subject	Re: 0 + not 0
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On 2015-07-11 17:02, Stefan Ram wrote:
> Serhiy Storchaka <storchaka@gmail.com> writes:
>>On 11.07.15 13:26, candide wrote:
>>>>>> 0 + not 0
>>>    File "<stdin>", line 1
>>>      0 + not 0
>>>            ^
>>> SyntaxError: invalid syntax
>>> What is syntactically wrong with 0 + not 0?
>>This looks as a bug to me. Please file a report
>
>    I look at Python 3.4.3:
>
> a_expr ::=  m_expr | a_expr "+" m_expr | a_expr "-" m_expr
>
>    So, »not 0« must be an »m_expr« when used as the right operand of »+«.
>
> m_expr ::=  u_expr | m_expr "*" u_expr | m_expr "//" u_expr | m_expr "/" u_expr | m_expr "%" u_expr
> u_expr ::=  power | "-" u_expr | "+" u_expr | "~" u_expr
> power ::=  primary ["**" u_expr]
> primary ::=  atom | attributeref | subscription | slicing | call
> atom      ::=  identifier | literal | enclosure
> enclosure ::=  parenth_form | list_display | dict_display | set_display | generator_expression | yield_atom
>
>    How can there be a »not«?
>
>    »not« is used in
>
> not_test ::=  comparison | "not" not_test
> and_test ::=  not_test | and_test "and" not_test
> or_test  ::=  and_test | or_test "or" and_test
> conditional_expression ::=  or_test ["if" or_test "else" expression]
> expression_nocond      ::=  or_test | lambda_expr_nocond
> expression             ::=  conditional_expression | lambda_expr
>
>    , but an »expression« is not an »m_expr«.
>
If "not" had the high priority of unary "-", then:

     not a < b

would be parsed as:

     (not a) < b

If you extended the OP's example to:

     0 + not 0 + 0

and permitted "not" in that position, it wouldn't be parsed as:

     0 + (not 0) + 0

but as:

     0 + (not (0 + 0))

Thread

0 + not 0 candide <c.candide@laposte.net> - 2015-07-11 03:26 -0700
  Re: 0 + not 0 Chris Angelico <rosuav@gmail.com> - 2015-07-11 20:38 +1000
  Re: 0 + not 0 Irmen de Jong <irmen.NOSPAM@xs4all.nl> - 2015-07-11 12:38 +0200
    Re: 0 + not 0 Luuk <luuk@invalid.lan> - 2015-07-11 13:12 +0200
      Re: 0 + not 0 Chris Angelico <rosuav@gmail.com> - 2015-07-11 21:20 +1000
        Re: 0 + not 0 Luuk <luuk@invalid.lan> - 2015-07-11 13:30 +0200
          Re: 0 + not 0 candide <c.candide@laposte.net> - 2015-07-11 04:54 -0700
            Re: 0 + not 0 Chris Angelico <rosuav@gmail.com> - 2015-07-11 22:05 +1000
              Re: 0 + not 0 candide <c.candide@laposte.net> - 2015-07-11 06:22 -0700
        Re: 0 + not 0 candide <c.candide@laposte.net> - 2015-07-11 04:48 -0700
      Re: 0 + not 0 random832@fastmail.us - 2015-07-11 16:46 -0400
  Re: 0 + not 0 Serhiy Storchaka <storchaka@gmail.com> - 2015-07-11 16:38 +0300
    Re: 0 + not 0 candide <c.candide@laposte.net> - 2015-07-11 08:07 -0700
    Re: 0 + not 0 MRAB <python@mrabarnett.plus.com> - 2015-07-11 17:20 +0100
    Re: 0 + not 0 Ian Kelly <ian.g.kelly@gmail.com> - 2015-07-11 10:56 -0600
      Re: 0 + not 0 Gregory Ewing <greg.ewing@canterbury.ac.nz> - 2015-07-12 18:12 +1200
    Re: 0 + not 0 Mark Lawrence <breamoreboy@yahoo.co.uk> - 2015-07-11 21:05 +0100

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