On Dec 7, 5:39 pm, Benjamin Kaplan <benjamin.kap...@case.edu> wrote: > On Mon, Dec 7, 2009 at 11:10 AM, vsoler <vicente.so...@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 -- http://mail.python.org/mailman/listinfo/python-list
