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 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 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
An important distinction between Vyper and Python is that Vyper does not implicitly return None
at the end of a function if no return
statement is given. All functions 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.
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 happen 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"