strange python behaviour with mixing globals/parameters and function named 'top'

Question

The following code (not directly in an interpreter, but execute as file)

def top(deck):
    pass

def b():
    global deck

produces the error

SyntaxError: name 'deck' is local and global

on python2.6.4 and

SyntaxError: name 'deck' is parameter and global

on python 3.1

python2.4 seems to accept this code, so does the 2.6.4 interactive interpreter.

This is already odd; why is 'deck' conflicting if it's a global in one method and a parameter in the other?

But it gets weirder. Rename 'top' to basically anything else, and the problem disappears.

Can someone explain this behaviour? I feel like I'm missing something very obvious here. Is the name 'top' somehow affecting certain scoping internals?

Update

This indeed appears to be a bug in the python core. I have filed a bug report.

Answer 1

It looks like it is a bug in the symbol table handling. Python/symtable.c has some code that (although somewhat obfuscated) does indeed treat 'top' as a special identifier:

if (!GET_IDENTIFIER(top) ||
    !symtable_enter_block(st, top, ModuleBlock, (void *)mod, 0)) {
    PySymtable_Free(st);
    return NULL;
}

followed somewhat later by:

if (name == GET_IDENTIFIER(top))
    st->st_global = st->st_cur->ste_symbols;

Further up the file there's a macro:

#define GET_IDENTIFIER(VAR) \
    ((VAR) ? (VAR) : ((VAR) = PyString_InternFromString(# VAR)))

which uses the C preprocessor to initialise the variable top to an interned string with the name of the variable.

I think the symbol table must be using the name 'top' to refer to the top level code, but why it doesn't use something that can't conflict with a real variable I have no idea.

I would report it as a bug if I were you.