All ways of assigning to __debug__ are forbidden:

>>> __debug__ = 1
  File "<stdin>", line 1
SyntaxError: cannot assign to __debug__
>>> for __debug__ in []: pass
... 
  File "<stdin>", line 1
SyntaxError: cannot assign to __debug__
>>> with cm() as __debug__: pass
... 
  File "<stdin>", line 1
SyntaxError: cannot assign to __debug__
>>> class __debug__: pass
... 
  File "<stdin>", line 1
SyntaxError: cannot assign to __debug__
>>> def __debug__(): pass
... 
  File "<stdin>", line 1
SyntaxError: cannot assign to __debug__
>>> def foo(__debug__): pass
... 
  File "<stdin>", line 1
SyntaxError: cannot assign to __debug__
>>> import __debug__
  File "<stdin>", line 1
SyntaxError: cannot assign to __debug__

The only exception is the assignment operator.

>>> (__debug__ := 'spam')
'spam'
>>> globals()['__debug__']
'spam'

This looks like a bug.

An idea for the fix? a direction where I could find the solution? ;-)

You need to add for the target of the assignment operator the same check as for other targets.

ok, I will check for the assignment operator in the code.

maybe in the AST (Python-ast.c) but I think not

ok, I found the commit of @emily 8f59ee01be3d83d5513a9a3f654a237d77d80d9a

and will try to find the assignment operator :=

You should look in Python/ast.c. The naming convention follows some form of "named expression" (e.g. NamedExpr, ast_for_namedexpr).

I'll have more time to look later this week, but let me know if you have any questions.

Hi @emily

Thank you for your help.

In fact, I have started to read Python-ast.c and ast.c, they are the main files for this kind of operations.

Thank you again,

Look at compile.c. The code for restricting other assignments is located here.

When I started working on this I did not notice the extra comments here.
 
I will close my PR so Stéphane can do the PR.

Thank you Pablo, if I see you at PyCon, maybe we could drink a good beer ;-)

New changeset 3ad91673057d410bf9f8c53df6bb8aa18c4b68ca by Pablo Galindo (Stéphane Wirtel) in branch 'master':
bpo-36052: Raise a SyntaxError when assigning a value to __debug__ with := (GH-11958)
https://github.com/python/cpython/commit/3ad91673057d410bf9f8c53df6bb8aa18c4b68ca