An interface is a set of function definitions used to enable communication between smart contracts. A contract interface defines all of that contract’s externally available functions. By importing the interface, your contract now knows how to call these functions in other contracts.
Declaring and using Interfaces¶
Interfaces can be added to contracts either through inline definition, or by importing them from a separate file.
interface keyword is used to define an inline external interface:
interface FooBar: def calculate() -> uint256: view def test1(): nonpayable
The defined interface can then be used to make external calls, given a contract address:
@external def test(foobar: FooBar): foobar.calculate()
The interface name can also be used as a type annotation for storage variables. You then assign an address value to the variable to access that interface. Note that casting an address to an interface is possible, e.g.
foobar_contract: FooBar @external def __init__(foobar_address: address): self.foobar_contract = FooBar(foobar_address) @external def test(): self.foobar_contract.calculate()
nonpayable annotation indicates that the call made to the external contract will be able to alter storage, whereas the
pure call will use a
STATICCALL ensuring no storage can be altered during execution. Additionally,
payable allows non-zero value to be sent along with the call.
interface FooBar: def calculate() -> uint256: pure def query() -> uint256: view def update(): nonpayable def pay(): payable @external def test(foobar: FooBar): foobar.calculate() # cannot change storage foobar.query() # cannot change storage, but reads itself foobar.update() # storage can be altered foobar.pay(value=1) # storage can be altered, and value can be sent
Vyper offers the option to set the following additional keyword arguments when making external calls:
Specify gas value for the call
Specify amount of ether sent with the call
Specify a default return value if no value is returned
default_return_value parameter can be used to handle ERC20 tokens affected by the missing return value bug in a way similar to OpenZeppelin’s
safeTransfer for Solidity:
ERC20(USDT).transfer(msg.sender, 1, default_return_value=True) # returns True ERC20(USDT).transfer(msg.sender, 1) # reverts because nothing returned
skip_contract_check=True is used and the called function returns data (ex.:
x: uint256 = SomeContract.foo(skip_contract_check=True), no guarantees are provided by the compiler as to the validity of the returned value. In other words, it is undefined behavior what happens if the called contract did not exist. In particular, the returned value might point to garbage memory. It is therefore recommended to only use
skip_contract_check=True to call contracts which have been manually ensured to exist at the time of the call.
Interfaces are imported with
from ... import statements.
Imported interfaces are written using standard Vyper syntax. The body of each function is ignored when the interface is imported. If you are defining a standalone interface, it is normally specified by using a
@external def test1(): pass @external def calculate() -> uint256: pass
You can also import a fully implemented contract and Vyper will automatically convert it to an interface. It is even possible for a contract to import itself to gain access to its own interface.
import greeter as Greeter name: public(String) @external def __init__(_name: String): self.name = _name @view @external def greet() -> String: return concat("Hello ", Greeter(msg.sender).name())
import statements, you must include an alias as a name for the imported package. In the following example, failing to include
as Foo will raise a compile error:
import contract.foo as Foo
from ... import¶
from you can perform both absolute and relative imports. You may optionally include an alias - if you do not, the name of the interface will be the same as the file.
# without an alias from contract import foo # with an alias from contract import foo as Foo
Relative imports are possible by prepending dots to the contract name. A single leading dot indicates a relative import starting with the current package. Two leading dots indicate a relative import from the parent of the current package:
from . import foo from ..interfaces import baz
Searching For Interface Files¶
When looking for a file to import, Vyper will first search relative to the same folder as the contract being compiled. For absolute imports, it also searches relative to the root path for the project. Vyper checks for the file name with a
.vy suffix first, then
When using the command line compiler, the root path defaults to the current working directory. You can change it with the
$ vyper my_project/contracts/my_contract.vy -p my_project
In the above example, the
my_project folder is set as the root path. A contract cannot perform a relative import that goes beyond the top-level folder.
from vyper.interfaces import ERC20 implements: ERC20
You can see all the available built-in interfaces in the Vyper GitHub repo.
Implementing an Interface¶
You can define an interface for your contract with the
import an_interface as FooBarInterface implements: FooBarInterface
This imports the defined interface from the vyper file at
an_interface.json if using ABI json interface type) and ensures your current contract implements all the necessary external functions. If any interface functions are not included in the contract, it will fail to compile. This is especially useful when developing contracts around well-defined standards such as ERC20.
Interfaces that implement functions with return values that require an upper bound (e.g.
String), the upper bound defined in the interface represents the lower bound of the implementation. Assuming a function
my_func returns a value
String in the interface, this would mean for the implementation function of
my_func that the return value must have at least length 1. This behavior might change in the future.
Vyper has a built-in format option to allow you to make your own Vyper interfaces easily.
$ vyper -f interface examples/voting/ballot.vy # Functions @view @external def delegated(addr: address) -> bool: pass # ...
If you want to do an external call to another contract, Vyper provides an external interface extract utility as well.
$ vyper -f external_interface examples/voting/ballot.vy # External Contracts interface Ballot: def delegated(addr: address) -> bool: view def directlyVoted(addr: address) -> bool: view def giveRightToVote(voter: address): nonpayable def forwardWeight(delegate_with_weight_to_forward: address): nonpayable # ...
The output can then easily be copy-pasted to be consumed.