[英]python reference error in building trie tree
I want to build a trivial trie tree in python, and here is my code 我想在python中构建一个琐碎的trie树,这是我的代码
class Node:
def __init__(self,ref={},num=0):
self.ref = ref
self.num = num
def makeTrie(node,s):
node.ref.setdefault(s[0],Node())
if len(s) == 1:
node.ref[s[0]].num += 1
return
makeTrie(node.ref[s[0]],s[1:])
trie = Node()
makeTrie(trie,'abcd')
print trie.ref['d'].num
print trie.ref['a'].ref['b'].ref['c'].ref['d'].num
And I am very confused,the statement print trie.ref['d'].num
also have value!! 而且我很困惑,声明
print trie.ref['d'].num
也很有价值! But I don't know when I insert 'd' in trie ? 但是我不知道何时在trie中插入'd'吗? The code above does't just insert 'd' in
trie.ref['a'].ref['b'].ref['c']
上面的代码
trie.ref['a'].ref['b'].ref['c']
只是在trie.ref['a'].ref['b'].ref['c']
插入'd'
You've run into a problem with mutable default arguments , I think. 我认为您在使用可变默认参数时遇到了问题。
In the initializer for Node
, you have ref={}
. 在
Node
的初始化程序中,您具有ref={}
。 But {}
is a dict
and hence a mutable object. 但是
{}
是dict
,因此是可变对象。 As such, each call to Node()
calls the initializer, which mutates the same ref
dictionary. 这样,对
Node()
每次调用都会调用初始化器,该初始化器将更改相同的ref
字典。
Fix (I think): 解决(我认为):
class Node:
# vvvv
def __init__(self, ref=None, num=0):
if ref is None: # <--
ref = {} # <--
self.ref = ref
self.num = num
def makeTrie(node,s):
node.ref.setdefault(s[0],Node())
if len(s) == 1:
node.ref[s[0]].num += 1
return
makeTrie(node.ref[s[0]],s[1:])
trie = Node()
makeTrie(trie,'abcd')
try:
print(trie.ref['d'].num)
except KeyError:
print('KeyError occurred!')
print(trie.ref['a'].ref['b'].ref['c'].ref['d'].num)
Result: 结果:
KeyError occurred!
1
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