Variables and variable types

Lists and dictionaries are variable types. This means that you can add, remove and change their elements. In the topic about variables, it was explained that a variable is a label: several different variables can point to the same object.

Consider the following example:

list1 = [1, 2, 3]
list2 = list1

list2[2] = 10

print(list1)

The above code will print [1, 2, 10]. This is because list1 and list2 are different labels, but point to the same object. To make a copy of a list or dictionary, use the method copy:

list1 = [1, 2, 3] 
list2 = list1.copy()

list2 [2] = 10 

print(list1)

This time the program will print [1, 2, 3] because we have modified a copy of list1.

You can use the operator is to check if two variables point to the same object. Please try to type in the console:

list1 = [1, 2, 3]

list2 = list1

list2 is list1    # True

When we create a copy, the operator is will allow us to check that we are dealing with different objects. In turn, the comparison operator (==) will show that these lists are identical:

list1 = [1, 2, 3]

list2 = list1.copy()

list2 is list1    # False

list2 == list1    # True

The opposite of the operator is is is not.

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