2.1 列表的创建与操作 / Creating and Manipulating Lists中文列表是有序可变序列用方括号创建。索引从 0 开始负数索引从末尾倒数。English: Lists are ordered, mutable sequences created with square brackets. Indexing starts at 0; negative indices count from the end.pythonfruits [apple, banana, cherry] print(fruits[0]) # apple 第一个 / first print(fruits[-1]) # cherry 最后一个 / last print(fruits[1:3]) # [banana, cherry] 切片 / slicing fruits.append(date) # 末尾添加 / append at end fruits.insert(1, blueberry) # 指定位置插入 / insert at index fruits.remove(banana) # 按值删除 / remove by value last fruits.pop() # 弹出末尾 / pop last print(fruits) # [apple, blueberry, cherry] print(last) # date2.2 列表推导式 / List Comprehensions中文列表推导式用一行代码生成列表语法为[表达式 for 元素 in 可迭代对象 if 条件]。English: A list comprehension builds a list in one line:[expression for item in iterable if condition].pythonnumbers [1, 2, 3, 4, 5, 6] # 每个元素平方 / square each element squares [n ** 2 for n in numbers] print(squares) # [1, 4, 9, 16, 25, 36] # 只保留偶数 / keep only even numbers evens [n for n in numbers if n % 2 0] print(evens) # [2, 4, 6] # 带条件表达式 / with conditional expression labels [偶 / even if n % 2 0 else 奇 / odd for n in numbers] print(labels)2.3 字典的创建与遍历 / Creating and Iterating Dictionaries中文字典存储键值对键必须可哈希且唯一。用get()访问不存在的键可返回默认值避免KeyError。English: Dictionaries store key-value pairs. Keys must be hashable and unique. Useget()to avoidKeyErrorwhen a key may be missing.pythonstudent { name: Alice, age: 20, major: Computer Science } print(student[name]) # Alice print(student.get(gpa, N/A)) # N/A 默认值 / default student[gpa] 3.8 # 新增键 / add key student[age] 21 # 修改值 / update value # 遍历 / iteration for key, value in student.items(): print(f{key}: {value}) # 只遍历键 / iterate keys only print(list(student.keys()))2.4 条件判断 / Conditional Statements中文elif是 else if 的缩写。Python 用缩进表示代码块通常为 4 个空格。English:elifis short for else-if. Python uses indentation (typically 4 spaces) to define blocks.pythonscore 85 if score 90: grade A elif score 80: grade B elif score 70: grade C else: grade D print(f分数 / Score: {score}, 等级 / Grade: {grade})2.5 循环 / Loops中文for用于遍历可迭代对象while在条件为真时重复执行。break跳出循环continue跳过本次迭代。English:foriterates over an iterable;whilerepeats while a condition is true.breakexits the loop;continueskips the current iteration.python# for 循环 / for loop for i in range(1, 6): # 1 到 5 / 1 to 5 if i 3: continue # 跳过 3 / skip 3 print(i, end ) print() # 1 2 4 5 # while 循环 / while loop count 0 while count 3: print(fcount {count}) count 1 # 带 else 的循环 / loop with else for n in [2, 4, 6]: if n % 2 ! 0: print(发现奇数 / found odd) break else: print(全部是偶数 / all even)2.6 练习 / Exercise中文统计一段文本中每个单词出现的次数按次数降序输出前 3 个。English: Count word frequencies in a text and print the top 3 in descending order.pythontext the quick brown fox jumps over the lazy dog the fox words text.split() counter {} for word in words: counter[word] counter.get(word, 0) 1 # 按次数降序排序 / sort by count descending top3 sorted(counter.items(), keylambda x: x[1], reverseTrue)[:3] for word, count in top3: print(f{word}: {count})github.0voice