What are the basic operation methods of dictionary in Python?

Initialization

<div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:php;toolbar:false"># 最常用这种 my_object = { &quot;a&quot;: 5, &quot;b&quot;: 6 } # 如果你不喜欢写大括号和双引号： my_object = dict(a=5, b=6)</pre><div class="contentsignin">Copy after login</div></div>

Merge dictionary

<div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:php;toolbar:false">a = { &quot;a&quot;: 5, &quot;b&quot;: 5 } b = { &quot;c&quot;: 5, &quot;d&quot;: 5 } c = { **a, **b } #最简单的方式 assert c == { &quot;a&quot;: 5, &quot;b&quot;: 5, &quot;c&quot;: 5, &quot;d&quot;: 5 } # 合并后还要修改，可以这样： c = { **a, **b, &quot;a&quot;: 10 } assert c == { &quot;a&quot;: 10, &quot;b&quot;: 5, &quot;c&quot;: 5, &quot;d&quot;: 5 } b[&quot;a&quot;] = 10 c = { **a, **b } assert c == { &quot;a&quot;: 10, &quot;b&quot;: 5, &quot;c&quot;: 5, &quot;d&quot;: 5 }</pre><div class="contentsignin">Copy after login</div></div>

Dictionary comprehension

<div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:php;toolbar:false"># 使用字典推导式来删除 key a = dict(a=5, b=6, c=7, d=8) remove = set([&quot;c&quot;, &quot;d&quot;]) a = { k: v for k,v in a.items() if k not in remove } # a = { &quot;a&quot;: 5, &quot;b&quot;: 6 } # 使用字典推导式来保留 key a = dict(a=5, b=6, c=7, d=8) keep = remove a = { k: v for k,v in a.items() if k in keep } # a = { &quot;c&quot;: 7, &quot;d&quot;: 8 } # 使用字典推导式来让所有的 value 加 1 a = dict(a=5, b=6, c=7, d=8) a = { k: v+1 for k,v in a.items() } # a = { &quot;a&quot;: 6, &quot;b&quot;: 7, &quot;c&quot;: 8, &quot;d&quot;: 9 }</pre><div class="contentsignin">Copy after login</div></div>

Collections Standard Library

Collections is a built-in module in Python that has several useful dictionary subclasses, Can greatly simplify Python code. Two of the classes I use frequently are defaultdict and Counter. Furthermore, since it is a subclass of dict, it has standard methods like items(), keys(), values(), etc.

<div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:php;toolbar:false">from collections import Counter counter = Counter() #counter 可以统计 list 里面元素的频率 counter.update(['a','b','a'] #此时 counter = Counter({'a': 2, 'b': 1}) #合并计数 counter.update({ &quot;a&quot;: 10000, &quot;b&quot;: 1 }) # Counter({'a': 10002, 'b': 2}) counter[&quot;b&quot;] += 100 # Counter({'a': 10002, 'b': 102}) print(counter.most_common()) #[('a', 10002), ('b', 102)] print(counter.most_common(1)[0][0]) # =&gt; a</pre><div class="contentsignin">Copy after login</div></div>

defaultdict is also the nirvana of dict:

<div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:php;toolbar:false">from collections import defaultdict # 如果字典的 value 是 字典 a = defaultdict(dict) assert a[5] == {} a[5][&quot;a&quot;] = 5 assert a[5] == { &quot;a&quot;: 5 } # 如果字典的 value 是列表 a = defaultdict(list) assert a[5] == [] a[5].append(3) assert a[5] == [3] # 字典的 value 的默认值可以是 lambda 表达式 a = defaultdict(lambda: 10) assert a[5] == 10 assert a[6] + 1 == 11 # 字典里面又是一个字典，不用这个，你要做多少初始化操作？ a = defaultdict(lambda: defaultdict(dict)) assert a[5][5] == {}</pre><div class="contentsignin">Copy after login</div></div>

Dictionary to JSON

What we usually call JSON refers to JSON string, which is a string. Dict can be converted into a string in JSON format.

<div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:php;toolbar:false">import json a = dict(a=5, b=6) # 字典转 JSON 字符串 json_string = json.dumps(a) # json_string = '{&quot;a&quot;: 5, &quot;b&quot;: 6}' # JSON 字符串转字典 assert a == json.loads(json_string) # 字典转 JSON 字符串保存在文件里 with open(&quot;dict.json&quot;, &quot;w+&quot;) as f: json.dump(a, f) # 从 JSON 文件里恢复字典 with open(&quot;dict.json&quot;, &quot;r&quot;) as f: assert a == json.load(f)</pre><div class="contentsignin">Copy after login</div></div>

Dictionary to Pandas

<div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:php;toolbar:false">import pandas as pd # 字典转 pd.DataFrame df = pd.DataFrame([ { &quot;a&quot;: 5, &quot;b&quot;: 6 }, { &quot;a&quot;: 6, &quot;b&quot;: 7 } ]) # df = #ab # 056 # 167 # DataFrame 转回字典 a = df.to_dict(orient=&quot;records&quot;) # a = [ #{ &quot;a&quot;: 5, &quot;b&quot;: 6 }, #{ &quot;a&quot;: 6, &quot;b&quot;: 7 } # ] # 字典转 pd.Series srs = pd.Series({ &quot;a&quot;: 5, &quot;b&quot;: 6 }) # srs = # a5 # b6 # dtype: int64 # pd.Series 转回字典 a = srs.to_dict() # a = {'a': 5, 'b': 6}</pre><div class="contentsignin">Copy after login</div></div>

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