Where is my namespace?

[vsoler]

On Dec 7, 5:39 pm, Benjamin Kaplan <benjamin.kap... at case.edu> wrote:
> On Mon, Dec 7, 2009 at 11:10 AM, vsoler <vicente.so... at gmail.com> wrote:
> > I take the example from Mark Lutz's excellent book "Learning Python".
>
> > *** In nested1.py  I have:
> > X=99
> > def printer(): print X
>
> > *** In nested2.py  I have:
> > from nested1 import X, printer
> > X=88
> > printer()
>
> > What is amazing is that running nested2.py prints 99 and not 88.
>
> > My questions are:
>
> > 1. Using statement "from" instead of "import" should not create a
> > namespace, at least that's what I think. However, the printer()
> > function is able to find 99 which is residing in...  a namespace?
>
> It doesn't create a namespace. But printer is still in nested1. When
> you do a "from ... import ...", the objects are imported into the
> current namespace. However, they are different names for what is
> currently the same object. They still follow Python's object
> semantics. Here's a simplified overview of what happens
>
> you run nested2.py
>
> 1 namespace: nested2, nothing in it (except for the special stuff)
>
> you run "from nested1 import X, printer". This executes everything in
> nested1 (because def and class are just executable statements in
> Python) and binds X and printer in nested2 to the objects in nested1.
> Note that these are now different names for the same objects.
>
> nested1: X = 99, printer = <function in nested1>
> nested2: X = 99 printer = <function in nested1>
>
> you call X = 88. Because you are rebinding the object, this name
> (nested2.X) is the only one that gets changed. Rebinding doesn't touch
> the object in nested1.
>
> nested1: X=99, printer = <function in nested1>
> nested2: x=88 printer = <function in nested1>
>
> Notice how the name "printer" in nested2 still refers to an object in
> nested1. It is simply another reference, not a new object. When you
> call printer, it's still sitting in nested1 and only sees nested1's X.
>
> > 2. I have tried to access the 88 by qualification from nested2.py.
> > However, I cannot. If using "print nested1.X" in nested2.py I get an
> > error
>
> that's because when you do "from ... import ..." it doesn't import the
> module itself. There's a totally different behavior between from ...
> import and import. Here's what you're trying to do
>
> nested2.py :
>
> import nested1
>
> nested1.X = 88
> nested1.printer()
>
>
>
> > 3. Mark says: The from statement is really an assignment to names in
> > the importer's scope--a name-copy operation, not a name aliasing.   I
> > don't fully understand what he means. Could anybody explain?
>
> Like I showed before, when you change the unqualified "X" in nested2,
> the X in nested1 doesn't change. Using from import is identical to
> doing something like this
>
> a = 1
> b = a
> b = 5
> a == 1 #True
>
> a is a name that temporarily referred to the same object as a.
> However, when you reassign b, it makes the name "b" refer to a
> different object. It doesn't change the object that a and b both refer
> to. When you import using  from ... import, you end up with 2
> different names that, at the start refer to the same object. Mutation
> (if you're using a mutable object) will show up in both namespaces,
> since they are referring to the same object, but assignment does not.
>
> > Thank you very much for your time.
>
> > Vicente Soler
> > --
> >http://mail.python.org/mailman/listinfo/python-list
>
>

I appreciate the preciseness and clearness of your explanations. Thank
you very much indeed.

Vicente Soler