Control Structures


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

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.


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.

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

def add_seven_with_overloading(a: uint256, b: uint256 = 3):
    return a + b

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


For external functions with default arguments like def my_function(x: uint256, b: uint256 = 1) the Vyper compiler will generate N+1 overloaded function selectors based on N default arguments.

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:

def _times_two(amount: uint256, two: uint256 = 2) -> uint256:
    return amount * two

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


Since calling an internal function is realized by jumping to its entry label, the internal function dispatcher ensures the correctness of the jumps. Please note that for internal functions which use more than one default parameter, Vyper versions >=0.3.8 are strongly recommended due to the security advisory GHSA-ph9x-4vc9-m39g.


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.

def readonly():
    # this function cannot write to state

def send_me_money():
    # this function can receive ether

Functions default to nonpayable when no mutability decorator is used.

Functions marked with @view cannot call mutable (payable or nonpayable) functions. Any external calls are made using the special STATICCALL opcode, which prevents state changes at the EVM level.

Functions marked with @pure cannot call non-pure functions.

Re-entrancy Locks

The @nonreentrant decorator places a global nonreentrancy lock on a function. An attempt by an external contract to call back into any other @nonreentrant function causes the transaction to revert.

def make_a_call(_addr: address):
    # this function is protected from re-entrancy

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

Nonreentrancy locks work by setting a specially allocated storage slot to a <locked> value on function entrance, and setting it to an <unlocked> value on function exit. On function entrance, if the storage slot is detected to be the <locked> value, execution reverts.

You cannot put the @nonreentrant decorator on a pure function. You can put it on a view function, but it only checks that the function is not in a callback (the storage slot is not in the <locked> state), as view functions can only read the state, not change it.

You can view where the nonreentrant key is physically laid out in storage by using vyper with the -f layout option (e.g., vyper -f layout foo.vy). Unless it is overriden, the compiler will allocate it at slot 0.


A mutable function can protect a view function from being called back into (which is useful for instance, if a view function would return inconsistent state during a mutable function), but a view function cannot protect itself from being called back into. Note that mutable functions can never be called from a view function because all external calls out from a view function are protected by the use of the STATICCALL opcode.


A nonreentrant lock has an <unlocked> value of 3, and a <locked> value of 2. Nonzero values are used to take advantage of net gas metering - as of the Berlin hard fork, the net cost for utilizing a nonreentrant lock is 2300 gas. Prior to v0.3.4, the <unlocked> and <locked> values were 0 and 1, respectively.


Prior to 0.4.0, nonreentrancy keys took a “key” argument for fine-grained nonreentrancy control. As of 0.4.0, only a global nonreentrancy lock is available.

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: uint256
    sender: indexed(address)

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


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.




Function can only be called externally


Function can only be called within current contract


Function does not read contract state or environment variables


Function does not alter contract state


Function is able to receive Ether


Function cannot be called back into during an external call

if statements

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


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:


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, a dynamic array, or generated from the built-in 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(stop, bound=N):

Here, stop can be a variable with integer type, greater than zero. N must be a compile-time constant. i begins as zero and increments by one until it is equal to stop. If stop is larger than N, execution will revert at runtime. In certain cases, you may not have a guarantee that stop is less than N, but still want to avoid the possibility of runtime reversion. To accomplish this, use the bound= keyword in combination with min(stop, N) as the argument to range, like range(min(stop, N), bound=N). This is helpful for use cases like chunking up operations on larger arrays across multiple transactions.

Another use of range can be with START and STOP bounds.

for i in range(START, STOP):

Here, 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.

Finally, it is possible to use range with runtime start and stop values as long as a constant bound value is provided. In this case, Vyper checks at runtime that end - start <= bound. N must be a compile-time constant.

for i in range(start, end, bound=N):