Storage

The storage is a mapping from a key to a value. The number of keys is for all practical purposes infinite. The value can be up to 32 bytes. Every value is initialized with a 0.

This is like the SSD in your computer. Storage is non-volatile.

source: https://docs.alchemy.com/docs/smart-contract-storage-layout

We will represent the storage as a dictionary.

class KeyValue:
    def __init__(self): self.storage = {}
    
    def load (self, key)       : return self.storage[key]
    def store(self, key, value): self.storage[key] = value

Warm/Cold

It costs different amount of gas whether we access a warm or cold storage slot. A slot is said to be warm if it was access before. Otherwise it is cold. Accessing a slot that is cold costs more gas than accessing a warm slot.

We implement that logic by keeping track of a cache. When we load a storage slot we save its key in that cache. If a key is in that cache it is said to be warm.

class Storage(KeyValue):
    def __init__(self):
        super().__init__()
        self.cache = []
        
    def load(self, key):
        warm = True if key in self.cache else False
        if not warm: self.cache.append(key)
        if key not in self.storage: return 0x00
        return warm, super().load(key)

storage = Storage()

We store 420 in storage slot 1

storage.store(1, 420)

Notice how the first time retrieving something from storage slot 1 its cold

storage.load(1)

(False, 420)

Now storage slot 1 is warm

storage.load(1)

(True, 420)

Reading a random storage that was not set to any value will return 0 and not throw an exception.

storage.load(42069)

0