PyTeal provides several control flow expressions to chain together multiple expressions and to conditionally evaluate expressions.
Exiting the Program:
Return expression causes the program to exit immediately. It takes a single argument,
which is the success value that the program should have. For example,
the program to immediately fail, and
Return(Int(1)) causes the program to immediately
succeed. Since the presence of a
Return statement causes the program to exit, no operations
after it will be executed.
Seq expression can be used to create a sequence of multiple expressions. It takes a
single argument, which is a list of expressions to include in the sequence. For example:
Seq([ App.globalPut(Bytes("creator"), Txn.sender()), Return(Int(1)) ])
Seq expression will take on the value of its last expression. Additionally, all
expressions in a
Seq expression, except the last one, must not return anything (e.g.
TealType.none). This restriction is in place because intermediate values must not
add things to the TEAL stack. As a result, the following is an invalid sequence:
Seq([ Txn.sender(), Return(Int(1)) ])
If you must include an operation that returns a value in the earlier
part of a sequence, you can wrap the value in a
Pop expression to discard it. For example,
Seq([ Pop(Txn.sender()), Return(Int(1)) ])
If(test-expr, then-expr, else-expr)
test-expr is always evaludated and needs to be typed
If it results in a value greater than 0, then the
then-expr is evaluated.
else-expr is evaluated. Note that
evaluate to the same type (e.g. both
You may also invoke an
If expression without an
In this case,
then-expr must be typed
Assert expression can be used to ensure that conditions are met before continuing the
program. The syntax for
test-expr is always evaluated and must be typed
test-expr results in a value greater than 0, the program continues. Otherwise, the program
immediately exits and indicates that it encountered an error.
Assert(Txn.type_enum() == TxnType.Payment)
The above example will cause the program to immediately fail with an error if the transaction type is not a payment.
Cond expression chians a series of tests to select a result expression.
The syntax of Cond is:
Cond([test-expr-1, body-1], [test-expr-2, body-2], . . . )
test-expr is evaluated in order. If it produces 0, the paired
is ignored, and evaluation proceeds to the next
As soon as a
test-expr produces a true value (> 0),
body is evaluated to produce the value for this
If none of
test-expr s evaluates to a true value, the
Cond expression will
be evaluated to
err, a TEAL opcode that causes the runtime panic.
Cond expression, each
test-expr needs to be typed
body could be typed either
TealType.bytes. However, all
body s must have the same data type. Otherwise, a
TealTypeError is triggered.
Cond([Global.group_size() == Int(5), bid], [Global.group_size() == Int(4), redeem], [Global.group_size() == Int(1), wrapup])
This PyTeal code branches on the size of the atomic transaction group.