python - Python: copy of a variable

Question

Is there a way in to make copy of a variable so that when the value changes of variable 'a' it copies itself to variable 'b'?

Example

a='hello'

b=a     #.copy()  or a function that will make a copy

a='bye'

# Is there a way to make 
# 'b' equal 'a' without 
# doing 'b=a'

print a
print b

I am having a problem using the Tkinter library where I have checkbutton that has been stored in a list and I'm trying to get the variable that it holds.

But it takes around 5 lines of code to reach the variable.

Is there a way of holding a copy of the variable that changes when the checkbutton variable changes?

Answer 1

You're exploring how Python deals with references. Assignment is simply binding a reference to an object on the right hand side. So, this is somewhat trivial:

a = 'foo'
b = a
print b is a  #True -- They *are the same object*

However, as soon as you do:

b = 'bar'
b is a  #False -- they're not longer the same object because you assigned a new object to b

Now this becomes really interesting with objects which are mutable:

a = [1]
b = a
b[0] = 'foo'
print a  #What?? 'a' changed?

In this case, a changes because b and a are referencing the same object. When we make a change to b (which we can do since it is mutable), that same change is seen at a because they're the same object.

So, to answer your question, you can't do it directly, but you can do it indirectly if you used a mutable type (like a list) to store the actual data that you're carrying around.

---

This is something that is very important to understand when working with Python code, and it's not the way a lot of languages work, so it pays to really think about/research this until you truly understand it.

Answer 2

In short, with simple value assignments, you cannot do this. As you saw:

a=4
b=a
a=5

>>> print b
4

However, with mutable objects like lists, you can do this. As such:

a=[1]
b=a
a.append(2)

>>> print a
[1,2]
>>> print b
[1,2]

Answer 3

Depending on what you want to do, you might want to check the weakref module.

This allows you to have a primary object and then several copies that will become None as soon as the primary object is gone.

Answer 4

You are asking if it is possible to create a reference to a variable and hold in another variable. No, this is not possible. See e.g. Create a reference to a variable (similar to PHP's "=&")?

Answer 5

There is an ugly way to create an independent copy of a simple variable via a temporary list:

a = 6; 
b = [a][0]