Expressions such as

a + not b
a * not b
+ not b

• not b

raise a SyntaxError, for instance :

>>> 0 + not 0
  File "<stdin>", line 1
    0 + not 0
          ^
SyntaxError: invalid syntax
>>> - not 0
  File "<stdin>", line 1
    - not 0
        ^
SyntaxError: invalid syntax
>>>

if the not expression is wrapped in parenthesis, expected evaluation occurs:

>>> - not 0
  File "<stdin>", line 1
    - not 0
        ^
SyntaxError: invalid syntax
>>> 0 + (not 0)
1
>>> - (not 0)
-1
>>>

The problem has been first submitted in comp.lang.python :

https://groups.google.com/forum/?hl=fr#!topic/comp.lang.python/iZiBs3tcuak

suggesting a bug report.

"not not 0" is compiled successful, while "+ not 0", "- not 0", and "~ not 0" are rejected.

On Sat, Jul 11, 2015 at 03:23:53PM +0000, candide wrote:
> 
> New submission from candide:
> 
> Expressions such as
> 
> a + not b
> a * not b
> + not b
> - not b
> 
> raise a SyntaxError, for instance :
> 
> 
> >>> 0 + not 0
>   File "<stdin>", line 1
>     0 + not 0
>           ^
> SyntaxError: invalid syntax

That has been invalid syntax since Python 1.5, if not older. I don't
think that it needs to be changed.

[steve@ando ~]$ python1.5
Python 1.5.2 (#1, Aug 27 2012, 09:09:18)  [GCC 4.1.2 20080704 (Red Hat 
4.1.2-52)] on linux2
Copyright 1991-1995 Stichting Mathematisch Centrum, Amsterdam
>>> 0 + not 0
  File "<stdin>", line 1
    0 + not 0
          ^
SyntaxError: invalid syntax

It looks like perfectly valid syntax to me and I cannot see why it should be semantically rejected. I disagree that it shouldn't be changed. I think it's a (minor) bug that should eventually get fixed.

"not" has a lower priority than unary "-"; this:

not a < b

is parsed as:

not (a < b)

How would you parse:

0 + not 0 + 0

?

Would it be parsed as:

0 + not (0 + 0)

?

Similar remarks could apply to "yield":

0 + yield 0

which is also a syntax error.

Hmm, right. I see your point and also the analogy with "yield". I could live with (maybe) giving it a better error message suggesting to use parentheses for clarity and otherwise keeping it a SyntaxError.

Funny, I ran into this one or two days ago, when refactoring some code that used the bitwise exclusive-or operator, since there is no boolean exclusive or:

-if (x == a) ^ (y != b): ...
+aa = x == a
+bb = y == b
+if aa ^ not bb: ...

It is fairly clear what I wanted to do above, but with “is not” you would have to avoid ambiguity:

>>> "spam" is not "ham"  # Using “is not” operator
True
>>> "spam" is (not "ham")  # Two distinct operators
False

I think it would be too complicated to make unary “not” bind more tightly than “and” in some cases but not in others. How would you handle these cases?

a + not b and c
a ** not b ** c
a is not not b

The way I see it, there is no equivalent problem with the other unary operators: arithmetic (+, -), bitwise (~), and “await”. Await has higher priority than all other binary operators, so no problem there. The other three are equal with the highest priority binary operator, exponentiation (**):

>>> 2 ** -1 ** 2  # Evaluated right to left
0.5
>>> 2 ** (-1) ** 2  # Negation first
2
>>> (2 ** -1) ** 2  # Left-hand exponentiation first
0.25

BTW, in the operator precedence table <https://docs.python.org/dev/reference/expressions.html#operator-precedence\>, I think exponentiation should be in the same priority group as the arithmetic and bitwise unary operations. At the moment it says exponentiation has higher priority, but has a footnote saying this is reversed on the right-hand side of an exponentiation. This is unclear when applied to my example above.

BTW “yield” is not a fair comparison because its syntax is even stranger (due to originally being a “yield” statement):

def g():
    x = yield + 1  # Yield the value +1
    y = (yield) + 1  # Add 1 after yielding
    return (yield)  # Mandatory brackets

Reproduced on 3.11:

>>> 0 + not 0
  File "<stdin>", line 1
    0 + not 0
        ^^^
SyntaxError: invalid syntax
>>> - not 0
  File "<stdin>", line 1
    - not 0
      ^^^
SyntaxError: invalid syntax

I think the best outcome here is to refine the syntax error. Making it behave a bit better is going to be quite a pain because of how unary "-" and "not" work on the priority level in the grammar.

I also don't think that facilitating the concatenation of operators without parentheses is a good idea (for readability reasons).

I will prepare a PR to improve the syntax error

Fixed in #28170.
Thanks to everyone involved!