Which is the expected behavior trying to get an undefined mapping value?

Question

In ERC20 standard, _allowed is defined as:

mapping (address => mapping (address => uint256)) private _allowed;

and the getter function is defined as:

function allowance(address owner, address spender) public view returns (uint256) {
        return _allowed[owner][spender];
}

I'm testing the function, and calling allowance(address1, address2) I receive 0. Why is that happening? The keys address1, and address2 were not initialized in _allowed.

I was expecting a KeyError, like in another languages, (python, etc.).

Answer 1

As per the solidity docs:

You can think of mappings as hash tables, which are virtually initialised such that every possible key exists and is mapped to a value whose byte-representation is all zeros, a type’s default value.

And regarding the default values here, it states:

A variable which is declared will have an initial default value whose byte-representation is all zeros. The “default values” of variables are the typical “zero-state” of whatever the type is. For example, the default value for a bool is false. The default value for the uint or int types is 0. For statically-sized arrays and bytes1 to bytes32, each individual element will be initialized to the default value corresponding to its type. Finally, for dynamically-sized arrays, bytes and string, the default value is an empty array or string.

So you are getting default initialized value of uint256 i.e. 0.