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.
All functions must include exactly one visibility decorator.
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 (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.value. If you require these values within an internal function you must pass them as parameters.
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.
@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 ...
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 and cannot return any values.
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 and return values - 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)
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 (
- the amount of ETH sent (
- the gas provided (
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.
All functions must include one visibility decorator (
@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 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 1: pass will fail to compile with a type mismatch.
You can also include
else statements, to add more conditional statements and a body that executes when the conditionals are false:
if CONDITION: ... elif OTHER_CONDITION: ... else: ...
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
You can use
for to iterate through the values of any array variable:
foo: int128 = [4, 23, 42] for i in foo: ...
In the above, example, the loop executes three times with
i assigned the values of
23, and then
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]: ...
- You cannot iterate over a multi-dimensional array.
imust 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.
Ranges are created using the
range function. The following examples are valid uses of
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
for i in range(start, stop): ...
STOP are literal integers, with
STOP being a greater value than
i begins as
START and increments by one until it is equal to
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.