Mutable vs Immutable Data

Python maintains data in memory which are referenced by variables.

Immutable strings

• Consider the following statement:

  x = "Hello world"
  

• The memory model looks like this.

  Figure 1

             +-------------+
      x ---> | Hello world |
             +-------------+
  

  Python strings are immutable. Namely, there is no way to modify the content of the data referenced by x.

• If we want to change the string x = "Hello", we have to create a new string and update the reference.

             +-------------+
      x -+   | Hello world |
         |   +-------------+
         |
         |    +-------+
         +--> | Hello |
              +-------+
  

Mutable lists

• Python supports mutable lists.

  Consider:

  x = [1, 2, 3]
  y = x
  

• The memory model looks like Figure 1.

      y
      |
      |
      |      +---+---+---+
      +----->|   |   |   |
             | 1 | 2 | 3 |
      x ---->|   |   |   |
             +---+---+---+
  

• Unlike strings, if we want to change the value of x, Python allows in-place modification of x.

  x[0] += 10
  

• This makes an in-place modification:

      y
      |
      |
      |      +----+---+---+
      +----->|    |   |   |
             | 11 | 2 | 3 |
      x ---->|    |   |   |
             +----+---+---+
  

Side-effects of mutable data

Note that the side effect of x[0] += 10 is that y also gets updated.

>>> y
[11, 2, 3]