处理字符串
字符串是 Python 中最常用的数据类型之一，用于表示文本信息。
0x01. 创建字符串
Python 中可以使用单引号、双引号或三引号创建字符串。
# 单引号
s1 = 'hello world'
# 双引号
s2 = "hello world"
# 三引号：用于多行字符串
s3 = '''这是一个
多行字符串'''
s4 = """另一个
多行字符串"""
# 原始字符串（不转义）
path = r'C:\Users\name\Documents'
print(path) # C:\Users\name\Documents
# 转义字符
print('hello\nworld') # 换行
print('hello\tworld') # 制表符
print('hello\'world') # 转义单引号
print('hello\\world') # 转义反斜杠
0x02. f-string 格式化
Python 3.6+ 引入的 f-string 提供了简洁的字符串格式化方式。
name = 'Alice'
age = 25
score = 95.5
# 基本用法
greeting = f'Hello, {name}! You are {age} years old.'
print(greeting) # Hello, Alice! You are 25 years old.
# 表达式
print(f'{name} will be {age + 1} next year.') # Alice will be 26 next year.
# 数字格式化
print(f'Score: {score:.1f}') # Score: 95.5
print(f'Pi: {3.14159:.2f}') # Pi: 3.14
# 对齐和填充
print(f'{"left":<10}') # left (左对齐)
print(f'{"right":>10}') # right (右对齐)
print(f'{"center":^10}') # center (居中)
print(f'{"hello":*^10}') # **hello*** (填充字符)
# 千位分隔符
print(f'{1234567:,}') # 1,234,567
print(f'{1234567:,.2f}') # 1,234,567.00
# 日期格式化
from datetime import datetime
now = datetime.now()
print(f'{now:%Y-%m-%d %H:%M:%S}') # 2024-01-01 12:30:45
0x03. r-string 原始字符串
原始字符串不处理转义字符，常用于正则表达式和文件路径。
# 文件路径
path = r'C:\Users\name\Documents\file.txt'
print(path) # C:\Users\name\Documents\file.txt
# 正则表达式
import re
pattern = r'\d+'
result = re.findall(pattern, 'There are 3 cats and 4 dogs')
print(result) # ['3', '4']
# 对比：非原始字符串需要双反斜杠
path2 = 'C:\\Users\\name\\Documents\\file.txt'
print(path2 == path) # True
0x04. 字符串操作
索引和切片
s = 'Hello, World!'
# 索引（从 0 开始）
print(s[0]) # 'H'
print(s[-1]) # '!'
print(s[-2]) # 'd'
# 切片 [start:stop:step]
print(s[0:5]) # 'Hello'
print(s[7:]) # 'World!'
print(s[:5]) # 'Hello'
print(s[::2]) # 'Hlo ol!'
print(s[::-1]) # '!dlroW ,olleH' (反转字符串)
字符串拼接和重复
# 拼接
s1 = 'Hello'
s2 = 'World'
s3 = s1 + ', ' + s2 + '!'
print(s3) # Hello, World!
# 重复
s4 = 'ha' * 3
print(s4) # 'hahaha'
# join 方法（推荐用于大量字符串拼接）
words = ['Python', 'is', 'awesome']
sentence = ' '.join(words)
print(sentence) # 'Python is awesome'
# 字符串拼接性能比较
import time
# 慢：每次拼接创建新字符串
start = time.time()
result = ''
for i in range(10000):
result += str(i)
slow_time = time.time() - start
# 快：使用 join
start = time.time()
result = ''.join(str(i) for i in range(10000))
fast_time = time.time() - start
print(f'Slow: {slow_time:.4f}s, Fast: {fast_time:.4f}s')
字符串查找
s = 'Hello, World!'
# find() 和 index()
print(s.find('World')) # 7 (找到返回索引)
print(s.find('Python')) # -1 (未找到返回 -1)
print(s.index('World')) # 7 (找到返回索引)
# print(s.index('Python')) # ValueError (未找到抛出异常)
# rfind() 和 rindex()：从右侧开始查找
s2 = 'Hello, Hello, Hello!'
print(s2.rfind('Hello')) # 14
# count()：统计出现次数
print(s2.count('Hello')) # 3
# startswith() 和 endswith()
print(s.startswith('Hello')) # True
print(s.endswith('!')) # True
字符串替换
s = 'Hello, World!'
# replace()：替换所有匹配项
new_s = s.replace('World', 'Python')
print(new_s) # Hello, Python!
# 限制替换次数
s2 = 'aaa bbb aaa ccc aaa'
new_s2 = s2.replace('aaa', 'xxx', 2)
print(new_s2) # xxx bbb xxx ccc aaa
# translate()：使用映射表替换
table = str.maketrans('aeiou', '12345')
print('hello'.translate(table)) # 'h2ll4'
字符串分割
s = 'Hello, World!'
# split()：按分隔符分割
words = s.split(' ')
print(words) # ['Hello,', 'World!']
# 限制分割次数
parts = 'a-b-c-d-e'.split('-', 2)
print(parts) # ['a', 'b', 'c-d-e']
# splitlines()：按行分割
text = '''line1
line2
line3'''
lines = text.splitlines()
print(lines) # ['line1', 'line2', 'line3']
# partition()：分割成三部分
before, sep, after = 'Hello-World'.partition('-')
print(before) # 'Hello'
print(sep) # '-'
print(after) # 'World'
字符串大小写转换
s = 'Hello, World!'
print(s.upper()) # 'HELLO, WORLD!'
print(s.lower()) # 'hello, world!'
print(s.title()) # 'Hello, World!'
print(s.capitalize()) # 'Hello, world!'
print(s.swapcase()) # 'hELLO, wORLD!'
字符串判断
# 字符类型判断
print('hello'.isalpha()) # True (全是字母)
print('123'.isdigit()) # True (全是数字)
print('hello123'.isalnum()) # True (字母或数字)
print(' '.isspace()) # True (全是空白)
print('Hello'.isupper()) # False (全是大写)
print('hello'.islower()) # True (全是小写)
print('Hello'.istitle()) # True (标题格式)
# 前缀和后缀判断
print('Hello'.startswith('He')) # True
print('Hello'.endswith('lo')) # True
字符串填充和对齐
# 居中填充
print('hello'.center(11, '*')) # ***hello***
# 左填充
print('42'.zfill(5)) # '00042'
print('42'.rjust(5, '0')) # '00042'
# 右填充
print('hello'.ljust(10, '-')) # 'hello-----'
# 去除空白
s = ' hello '
print(s.strip()) # 'hello'
print(s.lstrip()) # 'hello '
print(s.rstrip()) # ' hello'
# 去除指定字符
s2 = '###hello###'
print(s2.strip('#')) # 'hello'
0x05. 字符串编码和解码
# 字符串编码为字节
text = '你好，世界'
encoded = text.encode('utf-8')
print(encoded) # b'\xe4\xbd\xa0\xe5\xa5\xbd\xef\xbc\x8c\xe4\xb8\x96\xe7\x95\x8c'
# 字节解码为字符串
decoded = encoded.decode('utf-8')
print(decoded) # '你好，世界'
# 其他编码方式
gbk_encoded = text.encode('gbk')
print(gbk_encoded)
# 编码错误处理
try:
'你好'.encode('ascii')
except UnicodeEncodeError as e:
print(f'编码错误: {e}')
# 使用 errors 参数处理错误
print('你好'.encode('ascii', errors='ignore')) # b''
print('你好'.encode('ascii', errors='replace')) # b'??'
print('你好'.encode('ascii', errors='xmlcharrefreplace')) # b'你好'
0x06. 字符串格式化（其他方式）
除了 f-string，还有其他格式化方式：
# % 格式化（旧式）
name = 'Alice'
age = 25
print('Hello, %s! You are %d years old.' % (name, age))
# format() 方法
print('Hello, {}! You are {} years old.'.format(name, age))
print('Hello, {0}! You are {1} years old.'.format(name, age))
print('Hello, {name}! You are {age} years old.'.format(name=name, age=age))
# 模板字符串
from string import Template
t = Template('Hello, $name! You are $age years old.')
print(t.substitute(name=name, age=age))
0x07. 字符串常量
import string
# 字母
print(string.ascii_letters) # 'abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ'
print(string.ascii_lowercase) # 'abcdefghijklmnopqrstuvwxyz'
print(string.ascii_uppercase) # 'ABCDEFGHIJKLMNOPQRSTUVWXYZ'
# 数字
print(string.digits) # '0123456789'
# 标点符号
print(string.punctuation) # '!"#$%&\'()*+,-./:;<=>?@[\\]^_`{|}~'
# 空白字符
print(string.whitespace) # ' \t\n\r\x0b\x0c'
0x08. 正则表达式基础
import re
text = 'Contact us at info@example.com or support@example.com'
# 查找所有邮箱地址
pattern = r'\b[A-Za-z0-9._%+-]+@[A-Za-z0-9.-]+\.[A-Z|a-z]{2,}\b'
emails = re.findall(pattern, text)
print(emails) # ['info@example.com', 'support@example.com']
# 搜索第一个匹配
match = re.search(r'\d+', 'There are 42 cats')
if match:
print(match.group()) # '42'
# 替换
new_text = re.sub(r'\d+', 'NUMBER', 'There are 42 cats and 13 dogs')
print(new_text) # 'There are NUMBER cats and NUMBER dogs'
# 分割
parts = re.split(r'\s+', 'Hello World Python')
print(parts) # ['Hello', 'World', 'Python']
参考
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