Mutation in Python

1. Mutation occurs when we change the value stored at a location. The simplest locations are variables.

   >>> x = 123
   >>> x
   123
   >>> y = x
   >>> y
   123
   >>> y = 456
   >>> y
   456
   >>> x
   123
   >>>
   

2. However we can also refer to locations in structures such as lists, tuples, etc.

   >>> x = [5,4,3,2,1]
   >>> x[2]
   3
   >>> y = (5,4,3,2,1)
   >>> y[3]
   2
   >>>
   

3. Remember that lists and dictionaries are mutable whereas tuples and strings are not.

4. We can mutate a value within a mutable structure:

   >>> number_string = 'zero one two three four five six seven eight nine'
   >>> number_list = number_string.split()
   >>> number_list
   ['zero', 'one', 'two', 'three', 'four', 'five', 'six', 'seven', 'eight', 'nine']
   >>> number_list[5]
   'five'
   >>> number_list[5] = 'vijf'
   >>> number_list
   ['zero', 'one', 'two', 'three', 'four', 'vijf', 'six', 'seven', 'eight', 'nine']
   

5. But note that structure is shared. Here nummer_lijst is assigned to the same structure as number_list. This meal that when part of number_list is mutated, that same part of nummer_lijst is also mutated.

   >>> number_list = number_string.split()
   >>> nummer_lijst = number_list
   >>> number_list
   ['zero', 'one', 'two', 'three', 'four', 'five', 'six', 'seven', 'eight', 'nine']
   >>> nummer_lijst
   ['zero', 'one', 'two', 'three', 'four', 'five', 'six', 'seven', 'eight', 'nine']
   >>> number_list[5] = 'vijf'
   >>> number_list
   ['zero', 'one', 'two', 'three', 'four', 'vijf', 'six', 'seven', 'eight', 'nine']
   >>> nummer_lijst
   ['zero', 'one', 'two', 'three', 'four', 'vijf', 'six', 'seven', 'eight', 'nine']
   >>>
   

6. This can be pictured as follows

   >>> l1 = [1,2,3]
   >>> l2 = l1
   >>> l3 = [1,2,3]
   >>> l1[2] = 'three'
   >>> l1
   [1, 2, 'three']
   >>> l2
   [1, 2, 'three']
   >>> l3
   [1, 2, 3]
   

   

7. This behaviour holds not only for lists but for all mutable Python structures and for structures in structures in structures…

8. Note however that the + and * operators on sequences always produce new sequences which do not share structure with their operands

   >>> l = [1,2,3]
   >>> m = l + l
   >>> m
   [1, 2, 3, 1, 2, 3]
   >>> n = l * 4
   >>> n
   [1, 2, 3, 1, 2, 3, 1, 2, 3, 1, 2, 3]
   >>> l[0] = 8
   >>> m
   [1, 2, 3, 1, 2, 3]
   >>> n
   [1, 2, 3, 1, 2, 3, 1, 2, 3, 1, 2, 3]
   >>> l
   [8, 2, 3]
   

9. You can check whether two structures are equal using the == operator, and whether they are identical using the is operator. Identity is a stricter notion that equality. If two objects are identical they are always equal.

   >>> l1 = [1,2,3]
   >>> l2 = l1
   >>> l3 = [1,2,3]
   >>> l1 is l2
   True
   >>> l1 is l3
   False
   >>> l1 == l2
   True
   >>> l1 == l3
   True
   

10. In fact, each Python object has a unique identity which you can access using the id function

    >>> id(l1)
    140737352275080
    >>> id(l2)
    140737352275080
    >>> id(l3)
    140737352276160