Python: when globals().update works and when it doesn't

Question

I am aware of the fact that updating locals() or globals() should be avoided if possible. As far as I understand, updating locals() in a function normally results in a NameError when referencing an updated variable, as already discussed here:

How can I load all keys from a dict as local variables, a better aproach?

My understanding is that the interpreter creates a static lookup table for local variables, and thus changes to locals() and not visible at runtime.

How about updating globals() then? This doesn't seem to be a terribly great idea either. Why does the following code fail:

def foo():
    globals().update({'a': 1})
    print a
    if False: a = 0

>>> foo()
UnboundLocalError: local variable 'a' referenced before assignment

while removing the never-to-be-executed if False: a = 0 assignment works?

If the interpreter creates a look up table for performance reasons, shouldn't it take statements that will not be executed during runtime into account?

Answer 1

This has nothing to do with your update to the globals dict specifically. You'd get exactly the same result if you did this:

a = 1

def foo():
    print a
    if False: a = 0

foo()

That's because any assignment to a variable that's not explicitly marked as global within a scope makes the variable local throughout that scope.