Check your understanding of the previous reading: which types are mutable or immutable, and what happens when you call a mutating method.

Question 1

Identify each of the following as mutable or immutable.

String

Strings are immutable: a string method like .lower() returns a new string instead of changing the original.

List

Lists are mutable: a method like .append() changes the list itself, in place.

Integer

Boolean

Question 2

True or false: calling a method always changes the value of the variable you called it on. For example, does the following code change the value of the variable name?

name = "Klaus"
name.lower()

Select one answer, then check.

True
Not quite. Strings are immutable: name.lower() builds and returns a new, lowercased string, but nothing assigns that new string back to name, so name is unchanged.
False
Correct. name stays "Klaus" after this code runs. Only mutable types (like lists) can be changed in place by a method call; for an immutable type like a string, you must assign the method's result back to the variable, e.g. name = name.lower().

Question 3

What value will the variable longer_list have after the following code runs?

longer_list = [1, 2]
longer_list = longer_list.append(3)

Select one answer, then check.

[1, 2, 3]
Not quite. .append(3) does add 3 to the list in place, but the second line then overwrites longer_list with whatever .append() returns, not with the list itself.
[1, 2]
Not quite. The list itself really is changed by .append(3) - the problem is what happens to the variable right after.
3
Not quite. .append() mutates the list and returns None, not the value that was appended.
None
Correct. .append() mutates the list in place and then returns None. Assigning that return value back to longer_list overwrites the (correctly updated) list with None.
[3, 1, 2]
Not quite. .append() always adds its argument to the end of the list, never the front.