Python to RPN converter

[Thomas Klemm]

The following function is not translated properly:

Code:

def f(a, b):
    return -(a + b)

This is the generated code for the HP-42S:

Code:

LBL "f"
XEQ 65          // reorder 2 params for storage
STO 00          // param: a
RDN
STO 01          // param: b
RDN
RCL 00          // a
CHS
RCL 01          // b
+
RTN             // return
LBL 50          // PyRPN Support Library of
"-Utility Funcs-"
RTN             // ---------------------------
LBL 65          // Reverse params. (a,b) -> (b,a)
X<>Y
RTN

However CHS should be executed after the addition of a and b.
This calculates -a + b instead.

Compare it with the disassembly of the function f:

Code:

  1           0 RESUME                   0

  2           2 LOAD_FAST                0 (a)
              4 LOAD_FAST                1 (b)
              6 BINARY_OP                0 (+)
             10 UNARY_NEGATIVE
             12 RETURN_VALUE

This is the abstract syntax tree of the expression -(a + b):

Code:

Module(
    body=[
        Expr(
            value=UnaryOp(
                op=USub(),
                operand=BinOp(
                    left=Name(id='a', ctx=Load()),
                    op=Add(),
                    right=Name(id='b', ctx=Load()))))],
    type_ignores=[])

It looks good to me.
Therefore, I assume that it is a problem in the method visit_UnaryOp of RecursiveRpnVisitor:

Code:

    def visit_UnaryOp(self, node):
        """
        op=USub(),
        operand=Num(n=1))],
        """
        if isinstance(node.op, ast.Not):
            self.visit(node.operand)
            self.visit(node.op)
        else:
            # for parsing e.g. -1
            self.visit(node.op)
            self.visit(node.operand)