第三章练习答案

00-objects-and-mutability

练习 1：预测输出

x = [1, 2]
y = [1, 2]
z = x
print(x == y, x is y, x is z)  # True False True

练习 2：别名陷阱

def append_twice(item, lst):
    lst = lst.copy()  # 创建副本
    lst.append(item)
    lst.append(item)
    return lst

numbers = [1, 2, 3]
result = append_twice(4, numbers)
print(numbers)  # [1, 2, 3]
print(result)   # [1, 2, 3, 4, 4]

练习 3：浅拷贝问题

# 正确写法
matrix = [[0] * 3 for _ in range(3)]
matrix[0][0] = 1
print(matrix)  # [[1, 0, 0], [0, 0, 0], [0, 0, 0]]

# 错误写法
bad_matrix = [[0] * 3] * 3  # 3个引用指向同一个列表
bad_matrix[0][0] = 1
print(bad_matrix)  # [[1, 0, 0], [1, 0, 0], [1, 0, 0]]

01-lists-and-tuples

练习 1：切片操作

lst = [0,1,2,3,4,5,6,7,8,9]
print(lst[::2])    # [0, 2, 4, 6, 8]（偶数索引）
print(lst[-3:])    # [7, 8, 9]（倒数3个）
print(lst[::-1])   # [9, 8, 7, 6, 5, 4, 3, 2, 1, 0]（反转）

练习 2：列表推导式

squares = [x**2 for x in range(1, 101)]
divisible = [x for x in range(1, 101) if x % 3 == 0 or x % 5 == 0]
lengths = [len(s) for s in ['hello', 'world', 'python']]

02-dicts-and-sets

练习 1：词频统计

def word_count(text):
    words = text.lower().split()
    freq = {}
    for word in words:
        freq[word] = freq.get(word, 0) + 1
    return freq

# 或用Counter
from collections import Counter
def word_count(text):
    return Counter(text.lower().split())

练习 2：字典合并

def merge_dicts(d1, d2):
    return {**d1, **d2}

03-strings-and-unicode

练习 1：字符串反转

def reverse_string(s):
    return s[::-1]

练习 5：回文判断

def is_palindrome(s):
    s = s.lower().replace(' ', '')
    return s == s[::-1]

06-datetime-and-timezone

练习 1：计算天数差

from datetime import datetime

def days_between(date1, date2):
    d1 = datetime.strptime(date1, '%Y-%m-%d')
    d2 = datetime.strptime(date2, '%Y-%m-%d')
    return abs((d2 - d1).days)

07-json-and-serialization

练习 1：配置文件

import json

config = {'host': 'localhost', 'port': 8080}

# 保存
with open('config.json', 'w') as f:
    json.dump(config, f, indent=2)

# 读取
with open('config.json', 'r') as f:
    loaded = json.load(f)

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