Why does a loop or list comprehension work to initialize an array, but individually initializing the elements does not?

Question

Why does initializing the array arr work when it is done as a list comprehension (I think that is what the following example is --not sure), but not when each array location is initialized individually?

For example, this works: (a)

arr=[] 
arr=[0 for i in range(5)]

but (b),

arr=[]
arr[0]=0
arr[1]=0

etc, doesn't.

Isn't the arr=[0 for i in range(5)] instruction essentially doing what is done in (b) above in one fell swoop?

I realize that array sizes need to be predefined (or allocated). So, I can understand something like

arr= [0]*5

or using numpy,

arr = np.empty(10, dtype=object)

work. However, I don't see how (a) preallocates the array dimension "ahead of time". How does python interpret (a) vs. (b) above?

Answer 1

Firstly, there is no point in declaring a variable if you rebind it later anyway:

arr = []  # <-- this line is entirely pointless
arr = [0 for i in range(5)]

Secondly, the two expressions

[0 for i in range(5)]
[0] * 5

create a new list object, whereas

arr[0] = 0

mutates an existing one, namely it wants to reassign the first element of arr. Since this doesn't exist, you will see an error. You could do instead:

arr = []
arr.append(0)
arr.append(0)

to fill an initially empty list incrementally.

Note that a Python list is not an Array in, let's say, the Java sense that it has a predefined size. It is more like an ArrayList.