The rational behind this behavior is that if an ABIReturnSubroutine returns a value, calling it will return an abi.ReturnedValue; but if the subroutine does not return a valid (has void return type), then it returns a simple Expr.
This union type is problematic for calling code, since abi.ReturnedValue and Expr behave completely differently. As of right now, calling must must cast the value to one of abi.ReturnedValue or Expr for it to pass type checking.
Solution
At the moment I can't think of a good solution for this. ABIReturnSubroutine instances really do return either abi.ReturnedValue or Expr, and I'm not aware of any tricks we can use in the Python type system to make it aware of this.
Problem
When calling a subroutine decorated with
ABIReturnSubroutine, the Python type annotation of the returned value isabi.ReturnedValue | Expr. Take a look at the definition ofABIReturnSubroutine.__call__: https://github.com/algorand/pyteal/blob/4c716b44819b9d529df98776423e74dac31883b1/pyteal/ast/subroutine.py#L663-L665The rational behind this behavior is that if an
ABIReturnSubroutinereturns a value, calling it will return anabi.ReturnedValue; but if the subroutine does not return a valid (has void return type), then it returns a simpleExpr.This union type is problematic for calling code, since
abi.ReturnedValueandExprbehave completely differently. As of right now, calling must must cast the value to one ofabi.ReturnedValueorExprfor it to pass type checking.Solution
At the moment I can't think of a good solution for this.
ABIReturnSubroutineinstances really do return eitherabi.ReturnedValueorExpr, and I'm not aware of any tricks we can use in the Python type system to make it aware of this.