Python assignment: by value or by reference?
I was really confused some time ago regarding the assignment method used by Python. Ok, let's take it one step at a time.
Assignment in Python is done by reference, not by value
This is correct, but let's take the following example:
- >>> a = 1
- >>> b = a
- >>> a = 2
- >>> b
- 1
- >>>
The expected result would be 2, not 1. Let's take the following example to make things even more strange:
- >>> list1 = [1, 2, 3]
- >>> list2 = list1
- >>> list2[0] = 100
- >>> list1
- [100, 2, 3]
- >>>
Well, which one is it? The answer is: integers are immutable, which means we can't change them. Let's analyze the first example: a = 1. Two things are being created: the object a and the value 1. Some a points to 1. Next, b = a. Object b is created and b points to the value of a.
Now, let's shake things up: a = 2. You'd think that the innitial value that a was pointing too will change. But, as integers are immutable, that doesn't happen. Instead, a new integer object is created. So now, a is now pointing at the new integer object specified, as for b, it points at at the same old object that a used to be pointing at.
Category: Python
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