from file import variable creates copies of variables

Question

If I

from file import variable

and the varable is changed in module file, the variables value is not updated. If I

import file

the varaible file.variable is updated.

Is there a way of selectively importing variables from a module, placing them in the modules local scope and having the updates reflected.

• David Robinson: What kind of variable is it? If an int, float, bool or a string, than it is immutable.
• Robert Jacobs: It Started as None. Then it was assigned a file handle.

Answer 1

No. In Python, all variables are references (pointers) to objects. Simple assignments in re-bind the names being assigned—they point them to different objects. That is, if you have two names, a and b, that point to the same object, say 42, and you assign b to a different object, say "Don't Panic", a still points to 42.

a = b = 42
b = "Don't Panic"
print a

This remains true whether the names involved are in the same module or different modules.

If you do not want this behavior, do not use an additional name for the same object. As you have found, you should import file and access it as file.variable rather than from file import variable. The latter is equivalent to:

import file
variable = file.variable
del file

So, under the hood, from ... import is doing an assignment of a new name. The name is the same as the name it already has, but it's in a different scope and is a new reference to the object.

Another possible solution is to use mutable objects so that you don't need to do simple assignments. For example, if a is a list, then you can modify a[0] and a[2] and so on all you like without ever changing what a points to. You can even reassign the whole content of the object without re-binding the name: a[:] = [1, 2, 3]. Since you are changing the object itself, not what any of the names point to, the changes can be seen through any name.

• abarnert: +1, but it's worth pointing out that from file import variable is also creating a new binding thismodule.variable to the same object bound to file.variable. (In other words, it's basically like doing import file; variable = file.variable; del file.) Because I think that's half of what the OP was confused by.
• kindall: Good suggestion, I have elaborated on that theme a bit.
• Robert Jacobs: So if a function import SelfFile then SelfFile.Var is equivalent to global Var.