用一个键值和没有对应的值在python中初始化一个字典

Initializing a dictionary in python with a key value and no corresponding values

提问by user2989027

I was wondering if there was a way to initialize a dictionary in python with keys but no corresponding values until I set them. Such as:

我想知道是否有办法在 python 中用键初始化字典，但在我设置它们之前没有相应的值。如：

Definition = {'apple': , 'ball': }

and then later i can set them:

然后我可以设置它们：

Definition[key] = something

I only want to initialize keys but I don't know the corresponding values until I have to set them later. Basically I know what keys I want to add the values as they are found. Thanks.

我只想初始化键，但我不知道相应的值，直到我以后必须设置它们。基本上我知道我想在找到值时添加哪些键。谢谢。

采纳答案by Scott Hunter

You could initialize them to None.

您可以将它们初始化为None.

回答by Hammer

you could use a defaultdict. It will let you set dictionary values without worrying if the key already exists. If you access a key that has not been initialized yet it will return a value you specify (in the below example it will return None)

你可以使用 defaultdict。它会让你设置字典值而不必担心键是否已经存在。如果您访问尚未初始化的键，它将返回您指定的值（在下面的示例中，它将返回 None）

from collections import defaultdict
your_dict = defaultdict(lambda : None)

回答by codegeek

Use the fromkeysfunction to initialize a dictionary with any default value. In your case, you will initialize with Nonesince you don't have a default value in mind.

使用该fromkeys函数用任何默认值初始化字典。在您的情况下，您将初始化为，None因为您没有考虑默认值。

empty_dict = dict.fromkeys(['apple','ball'])

this will initialize empty_dictas:

这将初始化empty_dict为：

empty_dict = {'apple': None, 'ball': None}

As an alternative, if you wanted to initialize the dictionary with some default value other than None, you can do:

作为替代方案，如果您想使用除 之外的其他默认值来初始化字典None，您可以执行以下操作：

default_value = 'xyz'
nonempty_dict = dict.fromkeys(['apple','ball'],default_value)

回答by creggnog

You can initialize the values as empty strings and fill them in later as they are found.

您可以将这些值初始化为空字符串，然后在找到它们时填充它们。

dictionary = {'one':'','two':''}
dictionary['one']=1
dictionary['two']=2

回答by Robert Lugg

Based on the clarifying comment by @user2989027, I think a good solution is the following:

根据@user2989027 的澄清评论，我认为一个好的解决方案如下：

definition = ['apple', 'ball']
data = {'orange':1, 'pear':2, 'apple':3, 'ball':4}
my_data = {}
for k in definition:
  try:
    my_data[k]=data[k]
  except KeyError:
    pass
print my_data

I tried not to do anything fancy here. I setup my data and an empty dictionary. I then loop through a list of strings that represent potential keys in my data dictionary. I copy each value from data to my_data, but consider the case where data may not have the key that I want.

我尽量不在这里做任何花哨的事情。我设置了我的数据和一个空字典。然后我循环遍历表示数据字典中潜在键的字符串列表。我将数据中的每个值复制到 my_data，但请考虑数据可能没有我想要的键的情况。

回答by Robert Lugg

It would be good to know what your purpose is, why you want to initialize the keys in the first place. I am not sure you need to do that at all.

最好知道您的目的是什么，为什么要首先初始化密钥。我不确定你是否需要这样做。

1) If you want to count the number of occurrences of keys, you can just do:

1）如果你想计算键出现的次数，你可以这样做：

Definition = {}
# ...
Definition[key] = Definition.get(key, 0) + 1

2) If you want to get None (or some other value) later for keys that you did not encounter, again you can just use the get() method:

2）如果您想稍后为您没有遇到的键获取 None （或其他值），您可以再次使用 get() 方法：

Definition.get(key)  # returns None if key not stored
Definition.get(key, default_other_than_none)

3) For all other purposes, you can just use a list of the expected keys, and check if the keys found later match those.

3）出于所有其他目的，您可以只使用预期键的列表，并检查稍后找到的键是否与这些键匹配。

For example, if you only want to store values for those keys:

例如，如果您只想存储这些键的值：

expected_keys = ['apple', 'banana']
# ...
if key_found in expected_keys:
    Definition[key_found] = value

Or if you want to make sure all expected keys were found:

或者，如果您想确保找到所有预期的键：

assert(all(key in Definition for key in expected_keys))

回答by Aktufono

q = input("Apple")
w = input("Ball")
Definition = {'apple': q, 'ball': w}