The documentation at https://docs.python.org/3/reference/simple_stmts.html says that:

"Names listed in a global statement must not be used in the same code block textually preceding that global statement"

But then later:

"CPython implementation detail: The current implementation does not enforce the two restrictions,
but programs should not abuse this freedom, as future implementations may enforce them..."

Code like this

def f():
    x = 1
    global x

gives SyntaxWarning for several releases, maybe it is time to make it a SyntaxError?