To explain “why”:
The +
operation adds the array elements to the original array. The array.append
operation inserts the array (or any object) into the end of the original array, which results in a reference to self in that spot (hence the infinite recursion).
The difference here is that the + operation acts specific when you add an array (it’s overloaded like others, see this chapter on sequences) by concatenating the element. The append-method however does literally what you ask: append the object on the right-hand side that you give it (the array or any other object), instead of taking its elements.
An alternative
Use extend()
if you want to use a function that acts similar to the + operator (as others have shown here as well). It’s not wise to do the opposite: to try to mimic append with the + operator for lists (see my earlier link on why).
Little history
For fun, a little history: the birth of the array module in Python in February 1993. it might surprise you, but arrays were added way after sequences and lists came into existence.