Stack

The stack is a very simple data structure. It is a list with one restrictions. You can only directly interact with the top of it.

You can push a new value to the top of the stack or you can pop an already existing value from the stack.

A stack of plates is a good metaphor. You can add a plate or remove a plate from the top. A stack works in the exact same way.

The EVM stack has a maximum capacity of 1024 items. Every item on the stack is at max a 256 bit value (32 bytes).

MAXIMUM_STACK_SIZE = 1024

We also throw an exception if we try to pop a value of a stack that is empty.

class Stack:
    def __init__(self): self.items = []
    def __str__ (self): 
        ws = []
        for i, item in enumerate(self.items[::-1]):
            if   i == 0                : ws.append(f"{item} <first")
            elif i == len(self.items)-1: ws.append(f"{item} <last") 
            else                       : ws.append(str(item))
        return "\n".join(ws)
        
    def push(self, value): 
        if len(self.items) == MAXIMUM_STACK_SIZE-1: raise Exception("Stack overflow")
        self.items.append(value)    
    
    def pop(self, n=-1):
        if len(self.items) < n: raise Exception("Stack overflow")
        del self.items[n]

We create a Stack

stack = Stack()

We push 3 values to the stack

stack.push(2)
stack.push(4)
stack.push(1)

print(stack)

1 <first
4
2 <last

We pop one of the stack. Which is 1.

stack.pop()

Now only 2 values are left on the stack. Notice how we removed 1. And 4 has become the first value.

print(stack)

4 <first
2 <last