如何在 Python 中初始化空列表字典? [英] How do I initialize a dictionary of empty lists in Python?

问题描述

我尝试以编程方式创建列表字典未能让我单独处理字典键.每当我创建列表字典并尝试附加到一个键时，所有列表都会更新.这是一个非常简单的测试用例:

My attempt to programmatically create a dictionary of lists is failing to allow me to individually address dictionary keys. Whenever I create the dictionary of lists and try to append to one key, all of them are updated. Here's a very simple test case:

data = {}
data = data.fromkeys(range(2),[])
data[1].append('hello')
print data

实际结果:{0: ['hello'], 1: ['hello']}

预期结果:{0: [], 1: ['hello']}

这是有效的

data = {0:[],1:[]}
data[1].append('hello')
print data

实际和预期结果:{0: [], 1: ['hello']}

为什么 fromkeys 方法没有按预期工作?

Why is the fromkeys method not working as expected?

推荐答案

将 [] 作为第二个参数传递给 dict.fromkeys() 给出了一个相当无用的结果 - 所有字典中的值将是 same 列表对象.

Passing [] as second argument to dict.fromkeys() gives a rather useless result – all values in the dictionary will be the same list object.

在 Python 2.7 或更高版本中，您可以改用字典理解:

In Python 2.7 or above, you can use a dicitonary comprehension instead:

data = {k: [] for k in range(2)}

在早期版本的 Python 中，您可以使用

In earlier versions of Python, you can use

data = dict((k, []) for k in range(2))

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