AST Example

gistfile1.py

class BinOp(Node):
    def __init__(self, left, op, right, lineno):
        self.op = op
        self.left = left
        self.right = right
        self.lineno = lineno

    def __str__(self):
        return 'BinOp({0}, {1}, {2})'.format(self.op, self.left, self.right)

    def emit(self, depth):
        print self.__str__()

    def type_check(self):
        left = self.left.type_check()
        right = self.right.type_check()

        # Used for addition and multiplication, but also for logical OR and AND. These items
        # return the same as their input.
        if self.op == '+' or self.op == '*':
            if left == right == ('int', []) or left == right == ('bool', []):
                return left
            else:
                raise TypecheckException("incompatible types to binary operator " + self.op, self.lineno)
        # Items that work with int and bool but return bool.
        elif self.op == '=' or self.op == '!=':
            if left == right == ('int', []) or left == right == ('bool', []):
                return 'bool', []
            else:
                raise TypecheckException("incompatible types to comparison operator", self.lineno)
        # Items that work with int but return bool.
        elif  (self.op == '>' or
              self.op == '<' or
              self.op == '>=' or
              self.op == '<='):
            if left == right == ('int', []):
                return 'bool', []
            else:
                raise TypecheckException("incompatible types to binary operator " + self.op, self.lineno)
        # Items that work with int and return int.
        elif self.op == '/' or self.op == '%' or self.op == '-':
            if left == right == ('int', []):
                return left
            else:
                raise TypecheckException("incompatible types to binary operator", self.lineno)
                
...                

class If(Node):
    def __init__(self, test, body, orelse, lineno):
        self.test = test
        self.body = body
        self.orelse = orelse
        self.lineno = lineno
        pass

    def __str__(self):
        return 'If({0}, {1}, {2}, {3})'\
        .format(self.test, self.body, self.orelse, self.lineno)

    def emit(self, depth):
        print self.__str__()

    def type_check(self):
        test = self.test.type_check()

        if test != ('bool', []):
            raise TypecheckException("if test is not bool", self.lineno)

        for s in self.body:
            s.type_check()

        if self.orelse is not None:
            self.orelse.type_check()