Unit ​

The sp.unit type in SmartPy represents the absence of a meaningful result or value. It corresponds to the unit type in Michelson.

The unit type is commonly used in contracts and in variants.

Using unit in contract parameters ​

On Tezos every contract has a parameter type. For example, you can call a contract of type sp.contract[sp.int] with an argument of type sp.int. This example calls a contract with this type:

sp.transfer(42, sp.mutez(0), contract)

But what if a contract doesn't require any arguments? In this case it can specify a parameter of type sp.unit, giving the contract the type sp.contract[sp.unit].

To call such a contract from SmartPy, pass the unit value, represented in SmartPy as (). For example, this code calls a contract that has the type sp.contract[sp.unit]:

sp.transfer((), sp.mutez(0), contract)

For more information about contracts and calling them, see Contracts and sp.transfer.

Using unit in variants ​

SmartPy variants can be used to represent alternative cases with extra information. For example this variant represents a shape that can be either a circle or a rectangle:

shape : type = sp.variant(
   Circle=sp.int,
   Rectangle=sp.pair[sp.int, sp.int]
)

In the previous example, a circle comes with only one number (its radius) and a rectangle carries two numbers (its length and width).

But what about cases that don't require any extra info? sp.unit to the rescue! Here is a type that models events coming from a physical sensor:

sensor_event : type = sp.variant(
    TemperatureChange=sp.int,
    HumidityChange=sp.int,
    MotionDetected=sp.unit
)

Temperature and humidity changes each come with a number, whereas for detected motion there is no extra information, hence sp.unit.