Assignment by reference

When we assign a variable to another variable, it is done by reference. This means that both variables will refer to the same object in memory. Let's look at an example:

In this example, we have two variables: `list1` and `list2`. We assign `list1` to `list2` using the equals sign. Now, both `list1` and `list2` refer to the same list object `[1, 2, 3]`.

What this means is that if we make changes to the list through one variable, it will affect the other variable as well. For example:

In this code, we use the `append()` method to add the value `4` to `list1`. If we print `list2` after this, we will see that it also contains `[1, 2, 3, 4]`. This is because `list2` is just another name for the same list object.

The same concept applies to other mutable objects in Python, such as dictionaries or sets. When you assign a mutable object to another variable, both variables will refer to the same object.

On the other hand, if you assign an immutable object (like a number or a string) to another variable, a new copy of the object is created. Here is an example:

In this code, we assign the value `5` to `x`, and then assign `x` to `y`. After that, we update `x` to have the value `10`. If we print `y` after this, it will still be `5`. This is because `y` is a separate copy of the original value, not a reference to `x`.