jacksoncode.github.io/tutorials/python/70.html at master · jacksoncode/jacksoncode.github.io

Repository navigation

History

100

101

102

103

104

105

106

107

108

109

110

111

112

113

114

115

116

117

118

119

120

121

122

123

124

125

126

127

128

129

130

131

132

133

134

135

136

137

138

139

140

141

142

143

144

145

146

147

148

149

150

151

152

153

154

155

156

157

158

159

160

161

162

163

164

165

166

167

168

169

170

171

172

173

174

175

176

177

178

179

180

181

182

183

184

185

186

187

188

189

190

191

192

193

194

195

196

197

198

199

200

201

202

203

204

205

206

207

208

209

210

211

212

213

214

215

216

217

218

219

220

221

222

223

224

225

226

227

228

229

230

231

232

233

234

235

236

237

238

239

240

241

242

243

244

245

246

247

248

249

250

251

252

253

254

255

256

257

258

259

260

261

262

263

264

265

266

267

268

269

<!DOCTYPE html>

<head>

<title>16.2. collections - CodeClub</title>

<style>

.tutorial-container { max-width: 960px; margin: 70px auto 40px; padding: 0 20px; }

.tutorial-sidebar { position: fixed; top: 70px; left: 20px; width: 260px; max-height: calc(100vh - 90px); overflow-y: auto; background: var(--bg-secondary, #fff); border-radius: 8px; padding: 16px; box-shadow: 0 2px 8px rgba(0,0,0,0.08); }

.tutorial-sidebar h3 { font-size: 16px; margin-bottom: 12px; color: var(--text-primary, #1a1a2e); }

.tutorial-sidebar a { display: block; padding: 6px 10px; color: var(--text-secondary, #666); text-decoration: none; font-size: 14px; border-radius: 4px; }

.tutorial-sidebar a:hover, .tutorial-sidebar a.active { background: var(--primary, #3b82f6); color: #fff; }

.tutorial-sidebar .sub-link { padding-left: 24px; font-size: 13px; }

.tutorial-content { margin-left: 300px; background: var(--bg-secondary, #fff); border-radius: 8px; padding: 32px; box-shadow: 0 2px 8px rgba(0,0,0,0.08); }

.tutorial-content h1 { font-size: 28px; margin-bottom: 20px; color: var(--text-primary, #1a1a2e); border-bottom: 2px solid var(--primary, #3b82f6); padding-bottom: 12px; }

.tutorial-content h2 { font-size: 22px; margin: 24px 0 16px; color: var(--text-primary, #1a1a2e); }

.tutorial-content h3 { font-size: 18px; margin: 20px 0 12px; }

.tutorial-content p { line-height: 1.8; margin: 12px 0; color: var(--text-secondary, #333); }

.tutorial-content pre { background: #1e1e2e; color: #cdd6f4; padding: 16px; border-radius: 8px; overflow-x: auto; margin: 16px 0; }

.tutorial-content code { font-family: 'Fira Code', 'Cascadia Code', monospace; font-size: 14px; }

.tutorial-content p code { background: #f1f5f9; padding: 2px 6px; border-radius: 4px; color: #e83e8c; }

.tutorial-content blockquote { border-left: 4px solid var(--primary, #3b82f6); padding: 12px 20px; margin: 16px 0; background: #f8fafc; border-radius: 0 8px 8px 0; }

.tutorial-content ul, .tutorial-content ol { padding-left: 24px; margin: 12px 0; }

.tutorial-content li { margin: 6px 0; line-height: 1.7; }

.tutorial-content img { max-width: 100%; border-radius: 8px; margin: 16px 0; }

.tutorial-nav { display: flex; justify-content: space-between; margin-top: 32px; padding-top: 20px; border-top: 1px solid var(--border, #e5e7eb); }

.tutorial-nav a { color: var(--primary, #3b82f6); text-decoration: none; font-weight: 500; }

.tutorial-nav a:hover { text-decoration: underline; }

.tutorial-index { max-width: 960px; margin: 70px auto 40px; padding: 0 20px; }

.tutorial-card { background: var(--bg-secondary, #fff); border-radius: 12px; padding: 24px; margin: 16px 0; box-shadow: 0 2px 8px rgba(0,0,0,0.08); transition: transform 0.2s; }

.tutorial-card:hover { transform: translateY(-2px); }

.tutorial-card h3 { margin: 0 0 8px; color: var(--text-primary, #1a1a2e); }

.tutorial-card p { color: var(--text-secondary, #666); margin: 0 0 12px; }

.tutorial-card .badge { display: inline-block; padding: 4px 10px; border-radius: 20px; font-size: 12px; font-weight: 600; margin-right: 8px; }

.badge-python { background: #3b82f6; color: #fff; }

.badge-git { background: #f97316; color: #fff; }

.badge-ai { background: #8b5cf6; color: #fff; }

@media (max-width: 768px) {

.tutorial-sidebar { display: none; }

.tutorial-content { margin-left: 0; }

}

</style>

</head>

<body>

<span>CodeClub</span>

</a>

</button>

</ul>

</div>

</nav>

<div class="tutorial-container" style="max-width: 960px; margin: 80px auto 40px; padding: 0 20px;"><div class="tutorial-sidebar"><h3>目录</h3><a href="0.html">1. 简介</a><a href="1.html">2. Python历史</a><a href="2.html">3. 安装Python</a><a href="3.html">3.1. Python解释器</a><a href="4.html">4. 第一个Python程序</a><a href="5.html">4.1. 使用文本编辑器</a><a href="6.html">4.2. 输入和输出</a><a href="7.html">5. Python基础</a><a href="8.html">5.1. 数据类型和变量</a><a href="9.html">5.2. 字符串和编码</a><a href="10.html">5.3. 使用list和tuple</a><a href="11.html">5.4. 条件判断</a><a href="12.html">5.5. 模式匹配</a><a href="13.html">5.6. 循环</a><a href="14.html">5.7. 使用dict和set</a><a href="15.html">6. 函数</a><a href="16.html">6.1. 调用函数</a><a href="17.html">6.2. 定义函数</a><a href="18.html">6.3. 函数的参数</a><a href="19.html">6.4. 递归函数</a><a href="20.html">7. 高级特性</a><a href="21.html">7.1. 切片</a><a href="22.html">7.2. 迭代</a><a href="23.html">7.3. 列表生成式</a><a href="24.html">7.4. 生成器</a><a href="25.html">7.5. 迭代器</a><a href="26.html">8. 函数式编程</a><a href="27.html">8.1. 高阶函数</a><a href="28.html">8.1.1. map/reduce</a><a href="29.html">8.1.2. filter</a><a href="30.html">8.1.3. sorted</a><a href="31.html">8.2. 返回函数</a><a href="32.html">8.3. 匿名函数</a><a href="33.html">8.4. 装饰器</a><a href="34.html">8.5. 偏函数</a><a href="35.html">9. 模块</a><a href="36.html">9.1. 使用模块</a><a href="37.html">9.2. 安装第三方模块</a><a href="38.html">10. 面向对象编程</a><a href="39.html">10.1. 类和实例</a><a href="40.html">10.2. 访问限制</a><a href="41.html">10.3. 继承和多态</a><a href="42.html">10.4. 获取对象信息</a><a href="43.html">10.5. 实例属性和类属性</a><a href="44.html">11. 面向对象高级编程</a><a href="45.html">11.1. 使用__slots__</a><a href="46.html">11.2. 使用@property</a><a href="47.html">11.3. 多重继承</a><a href="48.html">11.4. 定制类</a><a href="49.html">11.5. 使用枚举类</a><a href="50.html">11.6. 使用元类</a><a href="51.html">12. 错误、调试和测试</a><a href="52.html">12.1. 错误处理</a><a href="53.html">12.2. 调试</a><a href="54.html">12.3. 单元测试</a><a href="55.html">12.4. 文档测试</a><a href="56.html">13. IO编程</a><a href="57.html">13.1. 文件读写</a><a href="58.html">13.2. StringIO和BytesIO</a><a href="59.html">13.3. 操作文件和目录</a><a href="60.html">13.4. 序列化</a><a href="61.html">14. 进程和线程</a><a href="62.html">14.1. 多进程</a><a href="63.html">14.2. 多线程</a><a href="64.html">14.3. ThreadLocal</a><a href="65.html">14.4. 进程 vs. 线程</a><a href="66.html">14.5. 分布式进程</a><a href="67.html">15. 正则表达式</a><a href="68.html">16. 常用内建模块</a><a href="69.html">16.1. datetime</a><a href="70.html">16.2. collections</a><a href="71.html">16.3. argparse</a><a href="72.html">16.4. base64</a><a href="73.html">16.5. struct</a><a href="74.html">16.6. hashlib</a><a href="75.html">16.7. hmac</a><a href="76.html">16.8. itertools</a><a href="77.html">16.9. contextlib</a><a href="78.html">16.10. urllib</a><a href="79.html">16.11. XML</a><a href="80.html">16.12. HTMLParser</a><a href="81.html">16.13. venv</a><a href="82.html">17. 常用第三方模块</a><a href="83.html">17.1. Pillow</a><a href="84.html">17.2. requests</a><a href="85.html">17.3. chardet</a><a href="86.html">17.4. psutil</a><a href="87.html">18. 图形界面</a><a href="88.html">18.1. 海龟绘图</a><a href="89.html">19. 网络编程</a><a href="90.html">19.1. TCP/IP简介</a><a href="91.html">19.2. TCP编程</a><a href="92.html">19.3. UDP编程</a><a href="93.html">20. 电子邮件</a><a href="94.html">20.1. SMTP发送邮件</a><a href="95.html">20.2. POP3收取邮件</a><a href="96.html">21. 访问数据库</a><a href="97.html">21.1. 使用SQLite</a><a href="98.html">21.2. 使用MySQL</a><a href="99.html">21.3. 使用SQLAlchemy</a><a href="100.html">22. Web开发</a><a href="101.html">22.1. HTTP协议简介</a><a href="102.html">22.2. HTML简介</a><a href="103.html">22.3. WSGI接口</a><a href="104.html">22.4. 使用Web框架</a><a href="105.html">22.5. 使用模板</a><a href="106.html">23. 异步IO</a><a href="107.html">23.1. 协程</a><a href="108.html">23.2. 使用asyncio</a><a href="109.html">23.3. 使用aiohttp</a><a href="110.html">24. FAQ</a><a href="111.html">25. 期末总结</a></div><div class="tutorial-content"><h1>16.2. collections</h1><div class="tutorial-body"><p>我们知道<code>tuple</code>可以表示不变集合，例如，一个点的二维坐标就可以表示成：</p>

<pre class="hljs"><code class="language-plain">>>> p = (1, 2)

</code></pre>

<p>但是，看到<code>(1, 2)</code>，很难看出这个<code>tuple</code>是用来表示一个坐标的。</p>

<p>定义一个<code>class</code>又小题大做了，这时，<code>namedtuple</code>就派上了用场：</p>

<pre class="hljs"><code class="language-plain">>>> from collections import namedtuple

>>> Point = namedtuple('Point', ['x', 'y'])

>>> p = Point(1, 2)

>>> p.x

>>> p.y

</code></pre>

<p><code>namedtuple</code>是一个函数，它用来创建一个自定义的<code>tuple</code>对象，并且规定了<code>tuple</code>元素的个数，并可以用属性而不是索引来引用<code>tuple</code>的某个元素。</p>

<p>这样一来，我们用<code>namedtuple</code>可以很方便地定义一种数据类型，它具备tuple的不变性，又可以根据属性来引用，使用十分方便。</p>

<p>可以验证创建的<code>Point</code>对象是<code>tuple</code>的一种子类：</p>

<pre class="hljs"><code class="language-plain">>>> isinstance(p, Point)

True

>>> isinstance(p, tuple)

True

</code></pre>

<p>类似的，如果要用坐标和半径表示一个圆，也可以用<code>namedtuple</code>定义：</p>

<pre class="hljs"><code class="language-python"><span class="hljs-comment"># namedtuple('名称', [属性list]):</span>

Circle = namedtuple(<span class="hljs-string">'Circle'</span>, [<span class="hljs-string">'x'</span>, <span class="hljs-string">'y'</span>, <span class="hljs-string">'r'</span>])

</code></pre>

<h3>deque</h3>

<p>使用<code>list</code>存储数据时，按索引访问元素很快，但是插入和删除元素就很慢了，因为<code>list</code>是线性存储，数据量大的时候，插入和删除效率很低。</p>

<p><code>deque</code>是为了高效实现插入和删除操作的双向列表，适合用于队列和栈：</p>

<pre class="hljs"><code class="language-plain">>>> from collections import deque

>>> q = deque(['a', 'b', 'c'])

>>> q.append('x')

>>> q.appendleft('y')

>>> q

deque(['y', 'a', 'b', 'c', 'x'])

</code></pre>

<p><code>deque</code>除了实现list的<code>append()</code>和<code>pop()</code>外，还支持<code>appendleft()</code>和<code>popleft()</code>，这样就可以非常高效地往头部添加或删除元素。</p>

<h3>defaultdict</h3>

<p>使用<code>dict</code>时，如果引用的Key不存在，就会抛出<code>KeyError</code>。如果希望key不存在时，返回一个默认值，就可以用<code>defaultdict</code>：</p>

<pre class="hljs"><code class="language-plain">>>> from collections import defaultdict

>>> dd = defaultdict(lambda: 'N/A')

>>> dd['key1'] = 'abc'

>>> dd['key1'] # key1存在

'abc'

>>> dd['key2'] # key2不存在，返回默认值

'N/A'

</code></pre>

<p>注意默认值是调用函数返回的，而函数在创建<code>defaultdict</code>对象时传入。</p>

<p>除了在Key不存在时返回默认值，<code>defaultdict</code>的其他行为跟<code>dict</code>是完全一样的。</p>

<h3>OrderedDict</h3>

<p>使用<code>dict</code>时，Key是无序的。在对<code>dict</code>做迭代时，我们无法确定Key的顺序。</p>

<p>如果要保持Key的顺序，可以用<code>OrderedDict</code>：</p>

<pre class="hljs"><code class="language-plain">>>> from collections import OrderedDict

>>> d = dict([('a', 1), ('b', 2), ('c', 3)])

>>> d # dict的Key是无序的

{'a': 1, 'c': 3, 'b': 2}

>>> od = OrderedDict([('a', 1), ('b', 2), ('c', 3)])

>>> od # OrderedDict的Key是有序的

OrderedDict([('a', 1), ('b', 2), ('c', 3)])

</code></pre>

<p>注意，<code>OrderedDict</code>的Key会按照插入的顺序排列，不是Key本身排序：</p>

<pre class="hljs"><code class="language-plain">>>> od = OrderedDict()

>>> od['z'] = 1

>>> od['y'] = 2

>>> od['x'] = 3

>>> list(od.keys()) # 按照插入的Key的顺序返回

['z', 'y', 'x']

</code></pre>

<p><code>OrderedDict</code>可以实现一个FIFO（先进先出）的dict，当容量超出限制时，先删除最早添加的Key：</p>

<pre class="hljs"><code class="language-python"><span class="hljs-keyword">from</span> collections <span class="hljs-keyword">import</span> OrderedDict

<span class="hljs-keyword">class</span> <span class="hljs-title class_">LastUpdatedOrderedDict</span>(<span class="hljs-title class_ inherited__">OrderedDict</span>):

<span class="hljs-keyword">def</span> <span class="hljs-title function_">__init__</span>(<span class="hljs-params">self, capacity</span>):

<span class="hljs-built_in">super</span>(LastUpdatedOrderedDict, <span class="hljs-variable language_">self</span>).__init__()

<span class="hljs-variable language_">self</span>._capacity = capacity

<span class="hljs-keyword">def</span> <span class="hljs-title function_">__setitem__</span>(<span class="hljs-params">self, key, value</span>):

containsKey = <span class="hljs-number">1</span> <span class="hljs-keyword">if</span> key <span class="hljs-keyword">in</span> <span class="hljs-variable language_">self</span> <span class="hljs-keyword">else</span> <span class="hljs-number">0</span>

<span class="hljs-keyword">if</span> <span class="hljs-built_in">len</span>(<span class="hljs-variable language_">self</span>) - containsKey >= <span class="hljs-variable language_">self</span>._capacity:

last = <span class="hljs-variable language_">self</span>.popitem(last=<span class="hljs-literal">False</span>)

<span class="hljs-built_in">print</span>(<span class="hljs-string">'remove:'</span>, last)

<span class="hljs-keyword">if</span> containsKey:

<span class="hljs-keyword">del</span> <span class="hljs-variable language_">self</span>[key]

<span class="hljs-built_in">print</span>(<span class="hljs-string">'set:'</span>, (key, value))

<span class="hljs-keyword">else</span>:

<span class="hljs-built_in">print</span>(<span class="hljs-string">'add:'</span>, (key, value))

OrderedDict.__setitem__(<span class="hljs-variable language_">self</span>, key, value)

</code></pre>

<h3>ChainMap</h3>

<p><code>ChainMap</code>可以把一组<code>dict</code>串起来并组成一个逻辑上的<code>dict</code>。<code>ChainMap</code>本身也是一个dict，但是查找的时候，会按照顺序在内部的dict依次查找。</p>

<p>什么时候使用<code>ChainMap</code>最合适？举个例子：应用程序往往都需要传入参数，参数可以通过命令行传入，可以通过环境变量传入，还可以有默认参数。我们可以用<code>ChainMap</code>实现参数的优先级查找，即先查命令行参数，如果没有传入，再查环境变量，如果没有，就使用默认参数。</p>

<p>下面的代码演示了如何查找<code>user</code>和<code>color</code>这两个参数：</p>

<pre class="hljs"><code class="language-python"><span class="hljs-keyword">from</span> collections <span class="hljs-keyword">import</span> ChainMap

<span class="hljs-keyword">import</span> os, argparse

<span class="hljs-comment"># 构造缺省参数:</span>

defaults = {

<span class="hljs-string">'color'</span>: <span class="hljs-string">'red'</span>,

<span class="hljs-string">'user'</span>: <span class="hljs-string">'guest'</span>

}

<span class="hljs-comment"># 构造命令行参数:</span>

parser = argparse.ArgumentParser()

parser.add_argument(<span class="hljs-string">'-u'</span>, <span class="hljs-string">'--user'</span>)

parser.add_argument(<span class="hljs-string">'-c'</span>, <span class="hljs-string">'--color'</span>)

namespace = parser.parse_args()

command_line_args = { k: v <span class="hljs-keyword">for</span> k, v <span class="hljs-keyword">in</span> <span class="hljs-built_in">vars</span>(namespace).items() <span class="hljs-keyword">if</span> v }

<span class="hljs-comment"># 组合成ChainMap:</span>

combined = ChainMap(command_line_args, os.environ, defaults)

<span class="hljs-built_in">print</span>(<span class="hljs-string">'color=%s'</span> % combined[<span class="hljs-string">'color'</span>])

<span class="hljs-built_in">print</span>(<span class="hljs-string">'user=%s'</span> % combined[<span class="hljs-string">'user'</span>])

</code></pre>

<p>没有任何参数时，打印出默认参数：</p>

<pre class="hljs"><code class="language-plain">$ python3 use_chainmap.py

color=red

user=guest

</code></pre>

<p>当传入命令行参数时，优先使用命令行参数：</p>

<pre class="hljs"><code class="language-plain">$ python3 use_chainmap.py -u bob

color=red

user=bob

</code></pre>

<p>同时传入命令行参数和环境变量，命令行参数的优先级较高：</p>

<pre class="hljs"><code class="language-plain">$ user=admin color=green python3 use_chainmap.py -u bob

color=green

user=bob

</code></pre>

<h3>Counter</h3>

<p><code>Counter</code>是一个简单的计数器，例如，统计字符出现的个数：</p>

<pre class="hljs"><code class="language-plain">>>> from collections import Counter

>>> c = Counter('programming')

>>> for ch in 'programming':

... c[ch] = c[ch] + 1

...

>>> c

Counter({'g': 2, 'm': 2, 'r': 2, 'a': 1, 'i': 1, 'o': 1, 'n': 1, 'p': 1})

>>> c.update('hello') # 也可以一次性update

>>> c

Counter({'r': 2, 'o': 2, 'g': 2, 'm': 2, 'l': 2, 'p': 1, 'a': 1, 'i': 1, 'n': 1, 'h': 1, 'e': 1})

</code></pre>

<p><code>Counter</code>实际上也是<code>dict</code>的一个子类，上面的结果可以看出每个字符出现的次数。</p>

<p><code>collections</code>模块提供了一些有用的集合类，可以根据需要选用。</p>

<p><a href="use_collections.py">use_collections.py</a></p>

</div>

<a href="/books/python/built-in-modules/datetime/index.html"

onclick="gs_load_chapter('/books/python/built-in-modules/datetime/index.html',true);return false;"

class="inline-block my-2">

stroke="currentColor">

<path stroke-linecap="round" stroke-linejoin="round"

d="M18.75 19.5l-7.5-7.5 7.5-7.5m-6 15L5.25 12l7.5-7.5" />

</svg>

datetime

</a>

</div>

<a href="/books/python/built-in-modules/argparse/index.html"

onclick="gs_load_chapter('/books/python/built-in-modules/argparse/index.html',true);return false;"

class="inline-block my-2">

argparse

stroke="currentColor">

<path stroke-linecap="round" stroke-linejoin="round"

d="M11.25 4.5l7.5 7.5-7.5 7.5m-6-15l7.5 7.5-7.5 7.5" />

</svg>

</a></div><div style="display: flex; justify-content: space-between; margin-top: 32px; padding-top: 20px; border-top: 1px solid var(--border, #e5e7eb);"><a href="69.html" style="color: var(--primary, #3b82f6); text-decoration: none; font-weight: 500;">← 16.1. datetime</a><a href="71.html" style="color: var(--primary, #3b82f6); text-decoration: none; font-weight: 500;">16.3. argparse →</a></div><p style="margin-top: 16px; font-size: 13px; color: var(--text-secondary, #999);">原文：<a href="https://liaoxuefeng.com/books/python/built-in-modules/collections/index.html" style="color: var(--primary, #3b82f6);">廖雪峰 - 16.2. collections</a></p></div></div>

</div>

</footer>

</body>

</html>

FilesExpand file tree

70.html

Latest commit

History

70.html

File metadata and controls