我假设您的表达式树由许多对象组成,其类型对应于它的表达式类型。前任。或者,等于、字符串等。像这样的东西:
class OrExpression:
def __init__(self, left, right):
self.left = left
self.right = right
class EqualsExpression:
def __init__(self, left, right):
self.left = left
self.right = right
class Literal:
def __init__(self, value):
self.value = value
class Variable:
def __init__(self, name):
self.name = name
与您的示例等效的表达式如下所示:
e = OrExpression(
EqualsExpression(
Variable("element"),
Literal("Apple")
),
OrExpression(
EqualsExpression(
Variable("element"),
Literal("Orange")
),
EqualsExpression(
Variable("element"),
Literal("Duck")
)
)
)
您可以为每个类创建一个eval
针对给定上下文评估自身的方法。像这样:
class OrExpression:
def __init__(self, left, right):
self.left = left
self.right = right
def eval(self, variables):
return self.left.eval(variables) or self.right.eval(variables)
class EqualsExpression:
def __init__(self, left, right):
self.left = left
self.right = right
def eval(self, variables):
return self.left.eval(variables) == self.right.eval(variables)
class Literal:
def __init__(self, value):
self.value = value
def eval(self, variables):
return self.value
class Variable:
def __init__(self, name):
self.name = name
def eval(self, variables):
return variables[self.name]
然后您可以调用eval
并提供上下文。在您的示例中,您只需要传入element
.
print e.eval({"element": "Apple"})
print e.eval({"element": "Duck"})
print e.eval({"element": "Banana"})
结果:
True
True
False
但是,如果您不按类型区分表达类型怎么办?假设您的树由普通的旧节点组成,这些节点标识它们使用其value
属性的表达式类型。代码大致相同,只是使用单个单片开关盒,而不是单独的eval
方法。
class Node:
def __init__(self, value=None, *children):
self.value = value
self.children = children
def evalTree(t, variables):
if t.value == "Or":
return evalTree(t.children[0], variables) or evalTree(t.children[1], variables)
elif t.value == "Equals":
return evalTree(t.children[0], variables) == evalTree(t.children[1], variables)
elif t.value == "Literal":
return t.children[0].value
elif t.value == "Variable":
name = t.children[0].value
else:
raise Exception("Unrecognized node type")
t = Node("Or",
Node("Equals",
Node("Variable", Node("element")),
Node("Literal", Node("Apple"))
),
Node("Or",
Node("Equals",
Node("Variable", Node("element")),
Node("Literal", Node("Apple"))
),
Node("Equals",
Node("Variable", Node("element")),
Node("Literal", Node("Apple"))
)
)
)
print evalTree(t,{"element": "Apple"})
print evalTree(t,{"element": "Duck"})
print evalTree(t,{"element": "Banana"})
结果:
True
True
False