137 lines
4.0 KiB
Markdown
137 lines
4.0 KiB
Markdown
---
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title: Python defaultdict
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localeTitle: Python defaultdict
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---
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## Python defaultdict
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Dictionary是Python中最常用的数据结构之一。 字典是无序的项集合，我们通常将字符和值存储在字典中。 让我们看一下通常如何使用字典的几个例子。
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```python
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# dictionary declaration 1
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dict1 = dict()
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# dictionary declaration 2
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dict2 = {}
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# Add items to the dictionary
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# The syntax to add and retrieve items is same for either of the two objects we defined above.
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key = "X"
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value = "Y"
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dict1[key] = value
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# The dictionary doesn't have any specific data-type.
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# So, the values can be pretty diverse.
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dict1[key] = dict2
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```
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现在让我们看一些检索方法。
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```python
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# Since "X" exists in our dictionary, this will retrieve the value
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value = dict1[key]
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# This key doesn't exist in the dictionary.
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# So, we will get a `KeyError`
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value = dict1["random"]
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```
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### 避免KeyError：使用.get函数
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如果字典中不存在给定的键，Python将抛出`KeyError` 。 有一个简单的解决方法。让我们看一下如何使用the来避免`KeyError` 用于字典的内置`.get`函数。
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```python
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dict_ = {}
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# Some random key
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random_key = "random"
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# The most basic way of doing this is to check if the key
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# exists in the dictionary or not and only retrieve if the
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# key exists. Otherwise not.
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if random_key in dict_:
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print(dict_[random_key])
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else:
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print("Key = {} doesn't exist in the dictionary".format(dict_))
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```
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很多时候，当密钥不存在时，我们可以获得默认值。例如，何时 建立一个柜台。有一种更好的方法可以从字典中获取默认值 丢失密钥而不是依赖标准的`if-else` 。
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```python
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# Let's say we want to build a frequency counter for items in the following array
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arr = [1,2,3,1,2,3,4,1,2,1,4,1,2,3,1]
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freq = {}
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for item in arr:
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# Fetch a value of 0 in case the key doesn't exist. Otherwise, fetch the stored value
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freq[item] = freq.get(item, 0) + 1
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```
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因此， `get(<key>, <defaultval>)`是一个方便的操作，用于从字典中检索任何给定键的默认值。 当我们想要将可变数据结构作为值（例如`list`或`set`处理时，会出现此方法的问题。
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```python
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dict_ = {}
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# Some random key
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random_key = "random"
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dict_[random_key] = dict_.get(random_key, []).append("Hello World!")
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print(dict_) # {'random': None}
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dict_ = {}
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dict_[random_key] = dict_.get(random_key, set()).add("Hello World!")
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print(dict_) # {'random': None}
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```
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你看到了问题吗？
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新的`set`或`list`不会被分配到字典的密钥。我们应该分配一个新的`list`或一`set` 在缺少值的情况下键，然后分别`append`或`add` 。莱伊看了一个例子。
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```python
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dict_ = {}
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dict_[random_key] = dict_.get(random_key, set())
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dict_[random_key].add("Hello World!")
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print(dict_) # {'random': set(['Hello World!'])}. Yay!
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```
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### 避免KeyError：使用defaultdict
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这在大多数时候都适用。但是，有一种更好的方法可以做到这一点。一种更加`pythonic`方式。 `defaultdict`是内置dict类的子类。 `defaultdict`只是分配我们在缺少键时指定的默认值。那么，这两个步骤：
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```python
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dict_[random_key] = dict_.get(random_key, set())
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dict_[random_key].add("Hello World!")
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```
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现在可以合并为一个步骤。例如
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```python
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from collections import defaultdict
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# Yet another random key
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random_key = "random_key"
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# list defaultdict
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list_dict_ = defaultdict(list)
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# set defaultdict
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set_dict_ = defaultdict(set)
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# integer defaultdict
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int_dict_ = defaultdict(int)
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list_dict_[random_key].append("Hello World!")
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set_dict_[random_key].add("Hello World!")
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int_dict_[random_key] += 1
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"""
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defaultdict(<class 'list'>, {'random_key': ['Hello World!']})
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defaultdict(<class 'set'>, {'random_key': {'Hello World!'}})
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defaultdict(<class 'int'>, {'random_key': 1})
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"""
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print(list_dict_, set_dict_, int_dict_)
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```
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* * *
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[官方文件](https://docs.python.org/2/library/collections.html) |