Reference types do not fit into 32 bytes. Because of this, copying their value is not as feasible as with value types. Therefore only the location, i.e. the reference, of the data is passed.

Fixed-size Lists

Fixed-size lists hold a finite number of elements which belong to a specified type.

Lists can be declared with _name: _ValueType[_Integer]. Multidimensional lists are also possible.

# Defining a list
exampleList: int128[3]

# Setting values
exampleList = [10, 11, 12]
exampleList[2] = 42

# Returning a value
return exampleList[0]

Structs

Structs are custom defined types that can group several variables.

Struct types can be used inside mappings and arrays. Structs can contain arrays and other structs, but not mappings.

Struct members can be accessed via struct.argname.

Mappings

Mappings are hash tables that are virtually initialized such that every possible key exists and is mapped to a value whose byte-representation is all zeros: a type’s default value.

The key data is not stored in a mapping, instead its keccak256 hash used to look up a value. For this reason mappings do not have a length or a concept of a key or value being “set”.

Mapping types are declared as HashMap[_KeyType, _ValueType].

KeyType can be any base or bytes type. Mappings, interfaces or structs are not support as key types.
ValueType can actually be any type, including mappings.

# Defining a mapping
exampleMapping: HashMap[int128, decimal]

# Accessing a value
exampleMapping[0] = 10.1

Docs: https://vyper.readthedocs.io/en/stable/types.html#reference-types

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