インタプリタの挙動を示すための例

expression.py

class Expression:                                                
    def __add__(self, that):                                     
        return BinaryOp('+', self, that)                         
                                                                 
    def __sub__(self, that):                                     
        return BinaryOp('-', self, that)                         
                                                                 
    def __mul__(self, that):                                     
        return BinaryOp('*', self, that)                         
                                                                 
    def __div__(self, that):                                     
        self.__truediv__(that)                                   
                                                                 
    def __truediv__(self, that):                                 
        return BinaryOp('/', self, that)                         
                                                                 
                                                                 
class BinaryOp(Expression):                                      
    def __init__(self, op, lhs, rhs):                            
        self.op = op                                             
        self.lhs = lhs                                           
        self.rhs = rhs                                           
                                                                 
class Value(Expression):                                         
    def __init__(self, value):                                   
        self.value = value                                       
                                                                 
class Ref(Expression):                                           
    def __init__(self, var):                                     
        self.var = var                                           
                                                                 
class Set(Expression):                                           
    def __init__(self, var, value):                              
        self.var = var                                           
        self.value = value                                       
                                                                 
class Block(Expression):                                         
    def __init__(self, args):                                    
        self.args = args                                         
                                                                 
def v(value):                                                    
    return Value(value)                                          
                                                                 
def set(var, value):                                             
    return Set(var, value)                                       
                                                                 
def ref(var):                                                    
    return Ref(var)                                              
                                                                 
def block(args):                                                 
    return Block(args)                                           
                                                                 
def eval(e):                                                     
    environment = {}                                             
    def evalMain(e):                                             
        if isinstance(e, Value):                                 
            return e.value                                       
        elif isinstance(e, Ref):                                 
            return environment[e.var]                            
        elif isinstance(e, Set):                                 
            environment[e.var] = evalMain(e.value)               
            return environment[e.var]                            
        elif isinstance(e, BinaryOp):                            
            if e.op == '+':                                      
                return evalMain(e.lhs) + evalMain(e.rhs)         
            elif e.op == '-':                                    
                return evalMain(e.lhs) - evalMain(e.rhs)         
            elif e.op == '*':                                    
                return evalMain(e.lhs) * evalMain(e.rhs)         
            else:                                                
                return evalMain(e.lhs) // evalMain(e.rhs)        
        else:                                                    
            last = None                                          
            for arg in e.args:                                   
                last = evalMain(arg)                             
            return last                                          
    return evalMain(e)                                           
                                                                 
program = block([                                                
    set('a', (v(1) + v(2)) * v(3) / v(4)), # a = (1 + 2) * 3 / 4 
    set('b', ref('a') * ref('a'))                                
])                                                               
print(eval(program))