[Tutor] When is = a copy and when is it an alias

[eryksun]

On Mon, Jan 27, 2014 at 2:01 PM, Danny Yoo <dyoo at hashcollision.org> wrote:
> And variable binding itself can even have a slightly
> different meaning, depending on whether the surrounding context is a
> function definition or not, establishing a local or global variable
> binding.  Whew!

Name binding is local unless you tell Python otherwise by declaring a
name to be global (also nonlocal in 3.x). CPython, for example,
compiles top-level module code to use the STORE_NAME instruction,
which stores to the locals mapping of the current frame. If the name
is declared global, it instead uses STORE_GLOBAL.

Module-level code is flagged to create a frame for which locals and
globals are the same dict. You could, however, exec code using
separate dicts for locals and globals:

    src = r'''
    global x
    x = 0
    y = 1
    '''

    gvars, lvars = {}, {}
    exec src in gvars, lvars

    ####

    >>> sorted(gvars)
    ['__builtins__', 'x']

    >>> sorted(lvars)
    ['y']

CPython bytecode:

    >>> code = compile(src, '<str>', 'exec')
    >>> dis.dis(code)
      3           0 LOAD_CONST               0 (0)
                  3 STORE_GLOBAL             0 (x)

      4           6 LOAD_CONST               1 (1)
                  9 STORE_NAME               1 (y)
                 12 LOAD_CONST               2 (None)
                 15 RETURN_VALUE