Control Structures

Functions

Functions are executable units of code within a contract. Functions may only be declared within a contract’s module scope.

@external
def bid():
    ...

Functions may be called internally or externally depending on their visibility. Functions may accept input arguments and return variables in order to pass values between them.

Visibility

All functions must include exactly one visibility decorator.

External Functions

External functions (marked with the @external decorator) are a part of the contract interface and may only be called via transactions or from other contracts.

@external
def add_seven(a: int128) -> int128:
    return a + 7

A Vyper contract cannot call directly between two external functions. If you must do this, you can use an interface.

Internal Functions

Internal functions (marked with the @internal decorator) are only accessible from other functions within the same contract. They are called via the self object:

@internal
def _times_two(amount: uint256) -> uint256:
    return amount * 2

@external
def calculate(amount: uint256) -> uint256:
    return self._times_two(amount)

Internal functions do not have access to msg.sender or msg.value. If you require these values within an internal function you must pass them as parameters.

Mutability

You can optionally declare a function’s mutability by using a decorator. There are four mutability levels:

  • Pure: does not read from the contract state or any environment variables.

  • View: may read from the contract state, but does not alter it.

  • Nonpayable: may read from and write to the contract state, but cannot receive Ether.

  • Payable: may read from and write to the contract state, and can receive Ether.

@view
@external
def readonly():
    # this function cannot write to state
    ...

@payable
@external
def send_me_money():
    # this function can receive ether
    ...

Functions default to nonpayable when no mutability decorator is used.

Re-entrancy Locks

The @nonreentrant(<key>) decorator places a lock on a function, and all functions with the same <key> value. An attempt by an external contract to call back into any of these functions causes the transaction to revert.

@external
@nonreentrant("lock")
def make_a_call(_addr: address):
    # this function is protected from re-entrancy
    ...

You can put the @nonreentrant(<key>) decorator on a __default__ function but we recommend against it because in most circumstances it will not work in a meaningful way.

The __default__ Function

A contract can also have a default function, which is executed on a call to the contract if no other functions match the given function identifier (or if none was supplied at all, such as through someone sending it Eth). It is the same construct as fallback functions in Solidity.

This function is always named __default__. It must be annotated with @external. It cannot expect any input arguments.

If the function is annotated as @payable, this function is executed whenever the contract is sent Ether (without data). This is why the default function cannot accept arguments - it is a design decision of Ethereum to make no differentiation between sending ether to a contract or a user address.

event Payment:
    amount: int128
    sender: indexed(address)

@external
@payable
def __default__():
    log Payment(msg.value, msg.sender)

Considerations

Just as in Solidity, Vyper generates a default function if one isn’t found, in the form of a REVERT call. Note that this still generates an exception and thus will not succeed in receiving funds.

Ethereum specifies that the operations will be rolled back if the contract runs out of gas in execution. send calls to the contract come with a free stipend of 2300 gas, which does not leave much room to perform other operations except basic logging. However, if the sender includes a higher gas amount through a call instead of send, then more complex functionality can be run.

It is considered a best practice to ensure your payable default function is compatible with this stipend. The following operations will consume more than 2300 gas:

  • Writing to storage

  • Creating a contract

  • Calling an external function which consumes a large amount of gas

  • Sending Ether

Lastly, although the default function receives no arguments, it can still access the msg object, including:

  • the address of who is interacting with the contract (msg.sender)

  • the amount of ETH sent (msg.value)

  • the gas provided (msg.gas).

The __init__ Function

__init__ is a special initialization function that may only be called at the time of deploying a contract. It can be used to set initial values for storage variables. A common use case is to set an owner variable with the creator the contract:

owner: address

def __init__():
    self.owner = msg.sender

You cannot call to other contract functions from the initialization function.

Decorators Reference

All functions must include one visibility decorator (@external or @internal). The remaining decorators are optional.

Decorator

Description

@external

Function can only be called externally

@internal

Function can only be called within current contract

@pure

Function does not read contract state or environment variables

@view

Function does not alter contract state

@payable

Function is able to receive Ether

@nonreentrant(<unique_key>)

Function cannot be called back into during an external call

if statements

The if statement is a control flow construct used for conditional execution:

if CONDITION:
    ...

CONDITION is a boolean or boolean operation. The boolean is evaluated left-to-right, one expression at a time, until the condition is found to be true or false. If true, the logic in the body of the if statement is executed.

Note that unlike Python, Vyper does not allow implicit conversion from non-boolean types within the condition of an if statement. if 1: pass will fail to compile with a type mismatch.

You can also include elif and else statements, to add more conditional statements and a body that executes when the conditionals are false:

if CONDITION:
    ...
elif OTHER_CONDITION:
    ...
else:
    ...

for loops

The for statement is a control flow construct used to iterate over a value:

for i in <ITERABLE>:
    ...

The iterated value can be a static array, or generated from the builtin range function.

Array Iteration

You can use for to iterate through the values of any array variable:

foo: int128[3] = [4, 23, 42]
for i in foo:
    ...

In the above, example, the loop executes three times with i assigned the values of 4, 23, and then 42.

You can also iterate over a literal array, as long as a common type can be determined for each item in the array:

for i in [4, 23, 42]:
    ...

Some restrictions:

  • You cannot iterate over a multi-dimensional array. i must always be a base type.

  • You cannot modify a value in an array while it is being iterated, or call to a function that might modify the array being iterated.

Range Iteration

Ranges are created using the range function. The following examples are valid uses of range:

for i in range(STOP):
    ...

STOP is a literal integer greater than zero. i begins as zero and increments by one until it is equal to STOP.

for i in range(start, stop):
    ...

START and STOP are literal integers, with STOP being a greater value than START. i begins as START and increments by one until it is equal to STOP.

for i in range(a, a + N):
    ...

a is a variable with an integer type and N is a literal integer greater than zero. i begins as a and increments by one until it is equal to a + N.