在Python中处理未声明的dict键 [英] Handle undeclared dict key in Python

问题描述

  c = {：sample = > 1，另一个=> 2} 
  

我可以这样处理表：

  [c [：sample] .nil？ ，c [：另一个] .nil？ ，c [：not_in_list] .nil？] 
  

我试图在Python中做同样的事情。我创建了一个新的字典：

  c = {sample：1，另一个：2} 
  

我无法处理以下的零价值异常：

  c [not-in-dictionary] 
  

这个：

  c [：not_in_dictionery]不是无
  

它返回一个异常，而不是 False。我该如何处理？

解决方案

在您特定的情况下，您应该可以执行此操作，而不是与无：

  c 
  

$ b $中的not_in_dictionary b

如果你真的使用这个代码，它将不起作用：

  c [：not_in_dictionary]不是无
  

Python没有特殊的：关键字字典键;使用普通字符串。

---

Python中的普通行为是在请求缺少的键时引发异常，让你处理例外。

  d = {a ：2，c：3} 
 
 try：
 print d [b] 
除了KeyError：
 print我们没有b字典！ 
  

如果要获取无如果缺少值，您可以使用 dict 的 .get 方法返回值（没有默认情况下）如果密钥缺失。

  print d .jpg（a）＃打印2 
打印d.get（b）＃prints无
打印d.get（b，0）＃打印0 
  

只需检查一个键是否具有 dict 中的值，请使用或中的不在关键字中。

打印a在d＃中
打印c不在d＃False 
打印d不在d＃True 
  





---

Python包含一个模块，允许您定义在正常使用时返回默认值而不是错误的字典： collections.defaultdict 。你可以这样使用：

  import collections 
 
d = collections $ d 
 print d [b]＃无
打印d [b] ==无＃True 
打印bin d＃True 
  

请注意与在。当你第一次查找一个密钥时，它会将它指向默认值，所以现在它被认为是 dict 。

In my Ruby application I have a hash table:

c = {:sample => 1,:another => 2}

I can handle the table like this:

[c[:sample].nil? , c[:another].nil? ,c[:not_in_list].nil?]

I'm trying to do the same thing in Python. I created a new dictionary:

c = {"sample":1, "another":2}

I couldn't handle the nil value exception for:

c["not-in-dictionary"]

I tried this:

c[:not_in_dictionery] is not None

and it is returning an exception instead of False. How do I handle this?

解决方案

In your particular case, you should probably do this instead of comparing with None:

"not_in_dictionary" in c

If you were literally using this code, it will not work:

c[:not_in_dictionary] is not None

Python doesn't have special :keywords for dictionary keys; ordinary strings are used instead.

---

The ordinary behaviour in Python is to raise an exception when you request a missing key, and let you handle the exception.

d = {"a": 2, "c": 3}

try:
    print d["b"]
except KeyError:
    print "There is no b in our dict!"

If you want to get None if a value is missing you can use the dict's .get method to return a value (None by default) if the key is missing.

print d.get("a") # prints 2
print d.get("b") # prints None
print d.get("b", 0) # prints 0

To just check if a key has a value in a dict, use the in or not in keywords.

print "a" in d # True
print "b" in d # False
print "c" not in d # False
print "d" not in d # True

---

Python includes a module that allows you to define dictionaries that return a default value instead of an error when used normally: collections.defaultdict. You could use it like this:

import collections

d = collections.defaultdict(lambda: None)
print "b" in d # False
print d["b"] # None
print d["b"] == None # True
print "b" in d # True

Notice the confusing behaviour with in. When you look up a key for the first time, it adds it pointing to the default value, so it's now considered to be in the dict.

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