12.14. Passing Mutable Objects¶
Take a look at the following code example. Can you predict what will happen when you run it?
Use Show CodeLens to step through the code to see why the assignment to the formal parameter
double did not affect the argument
num. An assignment to a formal parameter inside a function never
affects the argument in the caller.
On the other hand, if you are passing a mutable object, such as a list, to a function, and the function alters the object’s state, that state change will be visible to the caller when the function returns. Take a look at the following example.
Try stepping through this in codelens to see what happens. The state of the list referenced by
lst is altered
changeit, and since
mylst is an alias for
mylst is affected by the actions taken by the function.
Look closely at this line:
lst = "Michigan"
That statement modifies the state of
lst by changing the value in slot 0. Although that line may appear to contradict the
statement above that “an assignment to a formal parameter inside a function never affects the argument in the caller,”
note that there is a difference between assigning to a slot of a list, and assigning to the list variable itself.
To see that difference, try changing that line to the following:
lst = ["Michigan", "Wolverines"]
Then, run again. This time,
mylist is not altered. To understand why, use CodeLens to step carefully through the code
and observe how the assignment to
lst causes it to refer to a separate list.
Take a moment to experiment some more with the
changeit function. Change the body of the function to the following:
Step through using CodeLens. You should see that
mylst is affected by this change, since the state of the list is altered.
Then, try again with this as the body:
lst = lst + ["Michigan Wolverines"]
Step through using CodeLens. Here, we create a new list using the concatenation operator, and
mylst is not affected by the change.
Understanding the techniques that functions can and cannot use to alter the state of mutable parameters is important. You may want to take some time to study the information on this page more thoroughly and play with the examples until you feel confident about your grasp of the material.
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