bad_things.py

import ast

def bad_idea(op):
    """
    When overcomplicated isn't complicated enough...
    """
    
    OP_TABLE = {
        'add': ast.Add,
        'sub': ast.Sub,
    }

    tree = ast.Interactive(body=[
        ast.FunctionDef(
            name=op if op in OP_TABLE else 'add',
            args=ast.arguments(
                args=[
                    ast.arg(arg='x', annotation=None),
                    ast.arg(arg='y', annotation=None)
                ],
                vararg=None,
                kwonlyargs=[],
                kw_defaults=[],
                kwarg=None,
                defaults=[]
            ),
            body=[ast.Return(
                value=ast.BinOp(
                    left=ast.Name(id='x', ctx=ast.Load()),
                    op=OP_TABLE.get(op, 'add')(),
                    right=ast.Name(id='y', ctx=ast.Load())
                )
            )],
            decorator_list=[],
            returns=None
        )]
    )

    tree = ast.fix_missing_locations(tree)
    namespace = {}
    exec(compile(tree, '<ast>', 'single'), {}, namespace)
    return namespace.get(op, 'add')

if __name__ == '__main__':
    print(bad_idea('add'))
    print(bad_idea('sub'))