说明无法分配给 Function 调用错误

Question

def func(lst):
    return lst[0]
 
 lst = [1,2,3]

 func(lst) = 5 #a

 lst[0] = 5 #b

Why does 'a' give me a "can't assign to function call" error and 'b' runs perfectly fine, even though they are saying the same thing. Please explain and/or provide alternatives to what I am trying to do. (Ignore faulty indentation)

Answer 1

When you do lst[0]=5 you are actually changing the first element of your list lst to 5. Therefore, you can assign the value 5.

However, when you do func(lst) for the algorithm, you are trying to give func(lst) a value 5, which is not doable.

However, what you can do is to check if func(lst) == 5 Which means, does the value given by func(lst) equal to 5?

Answer 2

What do you think it "should" mean, if you were allowed to do func(lst) = 5 ?

You seem to be fixated by the fact that, in this case, func(lst) happens to always equal lst[0] , and that you know this - but what would it mean if func(lst) where to, say, always return 2 ? Would you expect that to mean you could set 2 equal to 5 (or whatever else you assigned)? That's clearly nonsensical. And what if the function didn't return anything?

The only things you can assign to are variables, and to object properties (of which list indices are a special case). A function call returns neither of these, but a value . Variables and object properties hold (or contain ) a value, but they are not in themselves values. It makes sense to assign to one of these, but you can't assign to a value.

In particular the lst[0] that your function returns doesn't actually bear any relation to lst itself. That's just how your function calculates its value - it looks at the list its been passed and takes the first element. But once it has done that and found, say, 1 (as in your example), that value of 1 is just 1 - it is "detached" from the list.