In CPython 3.10 :

Python 3.10.0 (tags/v3.10.0:b494f59, Oct  4 2021, 19:00:18) [MSC v.1929 64 bit (AMD64)] on win32
Type "help", "copyright", "credits" or "license" for more information.
>>> @x = 123
  File "<stdin>", line 1
    @x = 123
     ^^^^^^^
SyntaxError: invalid syntax. Maybe you meant '==' or ':=' instead of '='?

but both suggestions lead to a SyntaxError :

>>> @x == 123
...
  File "<stdin>", line 2

    ^
SyntaxError: invalid syntax
>>> @x := 123
...
  File "<stdin>", line 2

    ^
SyntaxError: invalid syntax
>>>


Maybe an error message such as "cannot assign to decorator" would be more appropriate ?

I agree this is weird, but technically is not incorrect. For example, consider this:

>>> def foo(f):
...     return f
...

>>> @x = foo
  File "<stdin>", line 1
    @x = foo
     ^^^^^^^
SyntaxError: invalid syntax. Maybe you meant '==' or ':=' instead of '='?

but with ':=' it works:

>>> @x := foo
... def g():
...   ...
...


Same for using '==' (although much work):

>>> def foo(f):
...     return f
...

>>> class A:
...   def __eq__(self, other):
...        return foo
...

>>> @A() = A()
  File "<stdin>", line 1
    @A() = A()
     ^^^
SyntaxError: cannot assign to function call here. Maybe you meant '==' instead of '='?

But it works with '==':

>>> @A() == A()
... def g():
...    ...
...
>>> g
<function g at 0x10d4f3c20>

This shows two things:

* Is technically syntactically valid, but semantically invalid. Notice that the suggestion is not false: the parser tells you that if you use '==' or ':=' it won't be a SyntaxError, and that is true.
* There are ridiculous cases when this can succeed, and actually some where it does even make sense (':=').

I am therefore closing this as 'not a bug', but please, feel free to comment back if you disagree :)