检查给定的键是否已存在于字典中并将其递增

问题描述

给定一个字典，我如何知道该字典中的给定键是否已设置为非 None 值?

Given a dictionary, how can I find out if a given key in that dictionary has already been set to a non-None value?

也就是说，我想这样做:

I.e., I want to do this:

my_dict = {}

if (my_dict[key] != None):
  my_dict[key] = 1
else:
  my_dict[key] += 1

也就是说，如果已经有一个值，我想增加它，否则将它设置为 1.

I.e., I want to increment the value if there's already one there, or set it to 1 otherwise.

推荐答案

您正在寻找 collections.defaultdict(适用于 Python 2.5+).这个

You are looking for collections.defaultdict (available for Python 2.5+). This

from collections import defaultdict

my_dict = defaultdict(int)
my_dict[key] += 1

会做你想做的事.

对于常规 Python dicts，如果给定键没有值，则在访问 dict 时您将不会得到 None -- 将引发 KeyError.因此，如果您想使用常规的 dict，那么您将使用

For regular Python dicts, if there is no value for a given key, you will not get None when accessing the dict -- a KeyError will be raised. So if you want to use a regular dict, instead of your code you would use

if key in my_dict:
    my_dict[key] += 1
else:
    my_dict[key] = 1

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