visitor pattern【此方的呆毛吧】

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visitor pattern

from abc import ABCMeta, abstractmethod
class Expr(metaclass = ABCMeta):
pass
class NumberExpr(Expr):
def __init__(self, num):
self.number_ = num
def number(self):
return self.number_
class BinaryExpr(Expr):
def __init__(self, op, lhs, rhs):
self.op_ = op
self.lhs_ = lhs
self.rhs_ = rhs
def getOp(self):
return self.op_
def getLHS(self):
return self.lhs_
def getRHS(self):
return self.rhs_
class Visitor(metaclass = ABCMeta):
@abstractmethod
def visitNumberExpr(self, expr):
pass
@abstractmethod
def visitBinaryExpr(self, expr):
pass
def visitExpr(self, expr):
visit_method = getattr(self, 'visit' + expr.__class__.__name__)
visit_method(expr)
class Printer(Visitor):
def __init__(self, pretty = True):
self.pretty_ = pretty
def print(self, expr):
self.visitExpr(expr)
def visitNumberExpr(self, expr):
print(expr.number(), end = '')
def visitBinaryExpr(self, expr):
print('(', end = '')
self.visitExpr(expr.getLHS())
if self.pretty_:
print(' {0} '.format(expr.getOp()), end = '')
else:
print(expr.getOp(), end = '')
self.visitExpr(expr.getRHS())

送TA礼物

1楼2012-10-28 12:01回复

print(')', end = '')
class Evaluator(Visitor):
def __init__(self):
self.value_stack_ = []
def eval(self, expr):
self.visitExpr(expr)
return self.value_stack_[-1]
def visitNumberExpr(self, expr):
self.value_stack_.append(expr.number())
def visitBinaryExpr(self, expr):
self.visitExpr(expr.getLHS())
self.visitExpr(expr.getRHS())
lhs_value = self.value_stack_.pop()
rhs_value = self.value_stack_.pop()
eval_func = {
'+': lambda l, r: l + r,
'-': lambda l, r: l - r,
'*': lambda l, r: l * r,
'/': lambda l, r: l / r
}
if expr.getOp() in eval_func:
result = eval_func[expr.getOp()](lhs_value, rhs_value)
self.value_stack_.append(result);
else:
raise Exception('unknown binary operator: {0}'.format(expr.getOp()))
if __name__ == '__main__':
# 1 + 2 * 3 + 4
e1 = NumberExpr(2)
e2 = NumberExpr(3)
e3 = BinaryExpr('*', e1, e2)
e4 = NumberExpr(1)
e5 = BinaryExpr('+', e4, e3)
e6 = NumberExpr(4)
e7 = BinaryExpr('+', e5, e6)
printer = Printer()
printer.print(e7)
print(' = ', end = '')
evaluator = Evaluator()
print(evaluator.eval(e7))

2楼2012-10-28 12:01