Behaviour of print in functions

[Craig McLean]

I'm learning Python and I have run into something I can't explain.  I have
the following code

X = 99
def foo():
    print X
    X = 100
    print X

foo()

When I call foo it complains that local variable X is referenced before been
assigned.  However X is a global and I was under the impression that it
should realize that and resolve it automatically.

I know that if you add a "global X" as the first line of function foo then
this all works properly.

I also know that

X=99
def qux():
    print X

qux()

prints 99 as I would expect.

I'm assuming what is happening is that name resolutions use the LGB rule,
but assignments always create a new entry in the local namespace, without
checking to see if it is covering up a global object of the same name.  I'm
also assuming that this happens before any of the code in the function is
executed.

Am I correct?  Or is something else happening here.