import inspect

# Note that this is just a proxy to make the tests pass
class Command(list):
    def matches(self, expression):
        pass

class Interpreter:

    def __init__(self, command=None):
        self.command = command if command else Command()
        self.vars = {}

    def interpret(self, expression):
        try:
            return eval(expression, {}, self.vars)
        except NameError:
            if self.command.matches(expression):
                parameter_name = inspect.getargspec(self.command.execute)[0][0]

                if parameter_name == 'variables':
                    return self.command.execute(self.vars)
                else:
                    return self.command.execute([self.command, ])
            else:
                return 'null'
        except SyntaxError:
            l_value, r_value = expression.split('=')

            try:
                self.vars[l_value] = int(r_value)
            except ValueError:
                self.vars[l_value] = self.interpret(r_value)