Issue 40196: symtable.Symbol.is_local() can be True for global symbols

Consider this function:

def f():
	global e
	e = 1

When inspecting symbols with symtable, symbol 'e' will be global and local, whereas is_local() should return False. See the attached file for reproducing. It will output to stdout:

symbol 'e' in function scope: is_global() = True, is_local() = True
global scope: e = 1

see https://stackoverflow.com/a/61040435/1725562 for a proposed fix

That fix is not correct. For instance consider:

>>> code2 = """\
... def foo():
...    x = 42
...    def bar():
...       return -1
... """
>>> top.get_children()[0]
<Function SymbolTable for foo in ?>
>>> top = symtable.symtable(code2, "?", "exec")
>>> top.get_children()[0].lookup('x')._Symbol__scope == symtable.LOCAL
True

but if we return x from bar:

>>> code = """\
... def foo():
...    x = 42
...    def bar():
...       return x
... """
>>> import symtable
>>> top = symtable.symtable(code, "?", "exec")
>>> top.get_children()[0].lookup('x')._Symbol__scope == symtable.LOCAL
False

In symtable.Function.get_locals() symbols with scopes in (LOCAL, CELL) are selected. Also

>>> code = """\
... def foo():
...    x = 42
...    def bar():
...       return x
... """
>>> import symtable
>>> top = symtable.symtable(code, "?", "exec")
>>> top.get_children()[0].lookup('x')._Symbol__scope == symtable.CELL
True

So I guess this would be the correct fix then:

def is_local(self):
    return self.__scope in (LOCAL, CELL)

> In symtable.Function.get_locals() symbols with scopes in (LOCAL, CELL) are selected.

Thanks for pointing that out. I will simplify PR 19391.

New changeset 799d7d61a91eb0ad3256ef9a45a90029cef93b7c by Pablo Galindo in branch 'master':
bpo-40196: Fix a bug in the symtable when reporting inspecting global variables (GH-19391)
https://github.com/python/cpython/commit/799d7d61a91eb0ad3256ef9a45a90029cef93b7c

New changeset 717f1668b3455b498424577e194719f9beae13a1 by Miss Islington (bot) in branch '3.7':
bpo-40196: Fix a bug in the symtable when reporting inspecting global variables (GH-19391)
https://github.com/python/cpython/commit/717f1668b3455b498424577e194719f9beae13a1

New changeset 8bd84e7f79a6cc7670a89a92edba3015aa781758 by Miss Islington (bot) in branch '3.8':
bpo-40196: Fix a bug in the symtable when reporting inspecting global variables (GH-19391) (GH-19394)
https://github.com/python/cpython/commit/8bd84e7f79a6cc7670a89a92edba3015aa781758

nosy: + pablogsal