convert SyntaxTree to Python source code

def gen_source(self, ast, name, customize, is_lambda=False, returnNone=False):
        """convert SyntaxTree to Python source code"""

        rn = self.return_none
        self.return_none = returnNone
        old_name = self.name
        self.name = name
        # if code would be empty, append 'pass'
        if len(ast) == 0:
            self.println(self.indent, 'pass')
        else:
            self.customize(customize)
            if is_lambda:
                self.write(self.traverse(ast, is_lambda=is_lambda))
            else:
                self.text = self.traverse(ast, is_lambda=is_lambda)
                self.println(self.text)
        self.name = old_name
        self.return_none = rn