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The fact that optional parameters' default values are only evaluated once,

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can lead to some unintuitive outcomes.

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So, here, we define a function that has two arguments,

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a and L. L's default value is an empty list.

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Now, note again that because this value is only evaluated once,

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what that means is that we have an empty list that

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lives somewhere in the frames in objects as the default value for L. So,

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here after we defined our function f,

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then we first print out f called with the value one.

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Now, in the body of f we actually mutate the value of L. So, what that means,

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is that when we call f on line five we pass in one for

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a and L is going to be that empty list that we created right here.

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When we run line two to say L.append a then we append one

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to our empty list and now our list is a list that has one item whose value is one.

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Now, on line three we return that list L. But,

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the thing to note here is that even though this list is going to go away in CodeLens,

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it actually keeps its value of a list that has one item whose value is one.

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So, even though this disappears in CodeLens,

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by the time we get to line six this list still has the value one.

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So, now, on line six when we call f with a equals two,

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then you can see that L is this list that has one item in it already.

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So, now, on line two when we append a to our list we add two onto this list.

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So, we're going to say,

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list is now a list that has one and two in it.

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So, if we kept calling our function f to add new values to our list,

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then we would get a longer and longer list as our default value for L. So, here,

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by the time we get to line seven and we call f with three,

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then when we call this function our list L already has two values in it,

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and on line two we add three on as a third value in our list.

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So, the important thing to note again here is that as

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our default value gets mutated it affects feature calls to this function f. Now,

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an important distinction here,

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is distinguishing between lists that are different objects,

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but have the same value versus this list,

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which is the same object.

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So, for example, on lines eight and nine,

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we pass in two different values for L. On line eight,

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we pass in a list that has the string "Hello" as its one item.

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On line nine, we pass in a list that looks identical,

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but because these are separate expressions,

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then these are actually separate objects.

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What that means, is that by the time we get to line

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eight and we print out the value of f called with a equals four,

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and L equals the list with the item "Hello."

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Then, when we actually call L.append a,

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then we're mutating this list,

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and that has no effect on this list.

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So, here we're mutating this list to add four onto it,

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which you can see here,

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and then when we get to line nine,

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then we're mutating a different list.

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So, note again here that we have a list

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whose only value is "Hello" it doesn't have the four that we added on to this list.

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That's all for now, until next time.