Statements

Vyper’s statements are syntactically similar to Python, with some notable exceptions.

Control Flow

break

The break statement terminates the nearest enclosing for loop.

for i: uint256 in [1, 2, 3, 4, 5]:
    if i == a:
        break

In the above example, the for loop terminates if i == a.

continue

The continue statement begins the next cycle of the nearest enclosing for loop.

for i: uint256 in [1, 2, 3, 4, 5]:
    if i != a:
        continue
    ...

In the above example, the for loop begins the next cycle immediately whenever i != a.

pass

pass is a null operation — when it is executed, nothing happens. It is useful as a placeholder when a statement is required syntactically, but no code needs to be executed:

# this function does nothing (yet!)

@external
def foo():
    pass

return

return leaves the current function call with the expression list (or None) as a return value.

return RETURN_VALUE

If a function has no return type, it is allowed to omit the return statement, otherwise, the function must end with a return statement, or another terminating action such as raise.

It is not allowed to have additional, unreachable statements after a return statement.

Event Logging

log

The log statement is used to log an event:

log MyEvent(...)

The event must have been previously declared.

See Event Logging for more information on events.

Warning

The evaluation order of arguments passed to log is undefined. The compiler may evaluate them in any order. Therefore, arguments with side effects should be evaluated in separate statements before the log call to ensure predictable behavior.

Assertions and Exceptions

Vyper uses state-reverting exceptions to handle errors. Exceptions trigger the REVERT opcode (0xFD) with the provided reason given as the error message. When an exception is raised the code stops operation, the contract’s state is reverted to the state before the transaction took place and the remaining gas is returned to the transaction’s sender. When an exception happens in a sub-call, it “bubbles up” (i.e., exceptions are rethrown) automatically.

If the reason string is set to UNREACHABLE, an INVALID opcode (0xFE) is used instead of REVERT. In this case, calls that revert do not receive a gas refund. This is not a recommended practice for general usage, but is available for interoperability with various tools that use the INVALID opcode to perform dynamic analysis.

raise

The raise statement triggers an exception and reverts the current call.

raise "something went wrong"

The error string is not required. If it is provided, it is limited to 1024 bytes.

assert

The assert statement makes an assertion about a given condition. If the condition evaluates falsely, the transaction is reverted.

assert x > 5, "value too low"

The error string is not required. If it is provided, it is limited to 1024 bytes.

This method’s behavior is equivalent to:

if not cond:
    raise "reason"