Dynamically growing an array to implement a stack

[M. Hamed]

I have trouble with some Python concept. The fact that you can not
assign to a non-existent index in an array. For example:

a = [0,1]
a[2] =========> Generates an error

I can use a.append(2) but that always appends to the end. Sometimes I
want to use this array as a stack and hence my indexing logic would be
something like:

If you are already at the end (based on your stack pointer):
      use append() then index (and inc your pointer)
if not:
      index directly (and inc your stack pointer)

If feel that doing this everytime I want to add an element that I have
to check whether it exists or not is too much. Is there any simpler
way to do this?

I know I can do something like this:

a = numpy.zeros(MAX_STACK_SIZE)

but I don't want to predetermine the stack size.

Any help?

Thanks