Mutable versus Immutable Objects

Strings, lists, tuples, integers, float, dictionaries, and sets are all types of python objects with different properties and uses. When using an object in a program or in your terminal, your session assigns an Id to access the computer's memory. The Id will be unique each session and on each computer, so the actual number returned by the id() function is irrelevant. What is relevant is when that number changes. If the id changes as the value changes, the computer had to assign a new id to the immutable object. If the id stays the same, then it is a mutable object, because the value associated with the id can be changed without changing the id. Integers are immutable:

>>> x = 5

>>> y = x    # direct the reference of y to be the reference of x

>>> id(x) == id(y)    # x and y point to same reference

True

>>> x = 4    # change the value of x

>>> id(x) == id(y)    # the id of x changed when we changed the value

False

>>> x == y    # the value of y did not change with x

False

I'm the type of learner that skims through vocabulary lessons to get to the action, but understanding this next part will save you some headaches when trying to manipulate mutable objects. Look what happens when I try to do the same thing I just did to the integers, but now to a list:

>>> x = [5]

>>> y = x    # direct the reference of y to be the reference of x

>>> id(x) == id(y)    # x and y point to the same reference

True

>>> x.append(4)    #change x from [5] to [5, 4]

>>> id(x) == id(y)    # x and y still point to the same reference

True

>>> x == y    # y changed with x

True

>>> y

[5, 4]

>>> z = [5, 5]    

>>> id(z)    # will be a different number for everyone

3075316972L

>>> z.append(3)    #z is now [5, 5, 3]

>>> id(z)    # id is constant, the list is mutable

3075316972L

I stumbled upon this while using random.shuffle on a list, while wishing to keep a copy of the list in it's original form. As you can see by assigning x equal to y, the lists changed together. That was an ineffective way to make a copy because all I did was assign the same Id two different names. Try determining if the other object types are mutable or immutable. I don't want to spoil the fun for you.