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So far I've told you that mutation

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modifies the existing object.

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But you can't really see the difference from what we've done with strings.

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Where we'll really see the difference is when we introduce a new variable.

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Let's go back to the example we had before,

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and now we're going to add an extra assignment statement,

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and we'll introduce a new variable.

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Suppose we introduce the variable "q."

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And we assign p to q, and that means the value of p,

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which is the object that's this list,

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is now what q will refer to.

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The important thing here is after the assignment,

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p and q refer to the same list.

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Suppose we did an assignment statement

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to modify the value of one of the elements of q.

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Well, that will change element 4 of q,

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so this is element 4 of q.

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It will change that value

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to the new exclamation point.

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It also changed the value of p.

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Even though the assignment statement didn't include p,

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the fact that p and q refer to the same object

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means that it changed the value of p.

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To show you that things are different with strings,

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let's try that with a string.

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See if you can guess what happens when we try to use assignment

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to replace the first letter in the string.

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Let's run it to see what happens.

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And what we get is an error,

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and we get an error because the string is not mutable.

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There's no way to change the value of the string,

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and the error says there's no way to do assignment in a string,

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that that type of object, because it's immutable, does not support assignment.

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A key difference between mutable objects and immutable objects

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is once an object is mutable,

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then we have to worry about other variables that might refer to the same object.

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We can change the value of that object,

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and it affects not just the variable that we think we changed,

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it affects the value of other variables as well.

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Let's see an example of that.

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I've initialized p to the list containing the strings "Hello."

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Now I have an assignment that introduces the new variable q

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and assigns p to that variable.

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And we'll print out the values of p and q,

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and we'll see that both p and q contain the string Hello.

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But now let's change the value at position 0.

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Now we have an assignment that stores

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in the value at position 0 of p the letter y.

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This changes the value of p.

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What may be more surprising is this also changes the value of q.

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Even though we didn't use q in the assignment,

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it changed the value of q because q

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refers to the same object as p.