Python字符串拼接的常用方法和最佳实践

# 基本字符串拼接
str1 = "Hello"
str2 = "World"
result = str1 + " " + str2
print(result)  # 输出: Hello World

# 拼接变量和字符串
name = "Alice"
greeting = "Hello, " + name + "!"
print(greeting)  # 输出: Hello, Alice!

# 逐步构建字符串
message = "Python"
message += " is "
message += "awesome!"
print(message)  # 输出: Python is awesome!

# 拼接字符串列表
words = ["Python", "is", "powerful"]
sentence = " ".join(words)
print(sentence)  # 输出: Python is powerful

# 拼接大量字符串（高效）
numbers = [str(i) for i in range(1000)]
csv_line = ",".join(numbers)

# 格式化字符串字面值
name = "Bob"
age = 30
message = f"My name is {name} and I am {age} years old."
print(message)  # 输出: My name is Bob and I am 30 years old.

# 表达式支持
a = 5
b = 10
result = f"{a} + {b} = {a + b}"
print(result)  # 输出: 5 + 10 = 15

# 位置参数
message = "{} + {} = {}".format(3, 4, 3+4)
print(message)  # 输出: 3 + 4 = 7

# 命名参数
info = "Name: {name}, Age: {age}".format(name="Charlie", age=25)
print(info)  # 输出: Name: Charlie, Age: 25

# 使用三引号
long_text = (
    "This is a long text "
    "that spans multiple lines "
    "without newline characters."
)

# 使用反斜杠
sql_query = "SELECT * FROM users " \
            "WHERE age > 18 " \
            "ORDER BY name;"

import os

# 使用 os.path.join 处理路径
folder = "/home/user"
file = "document.txt"
path = os.path.join(folder, "documents", file)
print(path)  # 输出: /home/user/documents/document.txt

# 字节串拼接
byte1 = b"Hello"
byte2 = b"World"
byte_result = byte1 + b" " + byte2
print(byte_result)  # 输出: b'Hello World'

import timeit

# 测试 + 运算符
t1 = timeit.timeit('s = ""; s += "a"*1000', number=10000)

# 测试 join 方法
t2 = timeit.timeit('s = "".join(["a"]*1000)', number=10000)

# 测试 f-string
t3 = timeit.timeit('s = f"{""a""*1000}"', number=10000)

print(f"+= 用时: {t1:.5f} 秒")
print(f"join 用时: {t2:.5f} 秒")
print(f"f-string 用时: {t3:.5f} 秒")

# 自动类型转换
num = 42
text = "The answer is " + str(num)

# 使用 f-string 自动转换
text = f"The answer is {num}"

# 列表转字符串
fruits = ["apple", "banana", "cherry"]
fruit_str = ", ".join(fruits)
print(fruit_str)  # 输出: apple, banana, cherry

# 使用不同分隔符
data = ["2023", "07", "25"]
date_str = "-".join(data)
print(date_str)  # 输出: 2023-07-25

# 不推荐（低效）
result = ""
for i in range(10000):
    result += str(i)
    
# 推荐（高效）
parts = []
for i in range(10000):
    parts.append(str(i))
result = "".join(parts)

# 旧式格式化（不推荐）
"Name: %s, Age: %d" % ("Alice", 30)

# 新式格式化（推荐）
"Name: {}, Age: {}".format("Alice", 30)

# f-string（最佳）
name = "Alice"
age = 30
f"Name: {name}, Age: {age}"

# 使用生成器表达式
with open("large_file.txt") as f:
    big_string = "".join(line for line in f)

# 错误示例
age = 25
message = "Age: " + age  # TypeError: can only concatenate str to str

# 解决方案
message = "Age: " + str(age)
message = f"Age: {age}"

# 不同编码字符串拼接
str1 = "Hello".encode("utf-8")
str2 = "世界".encode("gbk")

# 解决方案：统一编码
str1_decoded = str1.decode("utf-8")
str2_decoded = str2.decode("gbk")
result = str1_decoded + str2_decoded

# 低效的循环拼接
s = ""
for i in range(100000):
    s += "a"  # 每次创建新字符串，O(n²)复杂度

# 高效解决方案
s = "".join("a" for _ in range(100000))  # O(n)复杂度

def smart_concat(*args, sep=" "):
    """
    智能拼接任意数量参数
    
    参数:
        *args: 要拼接的值
        sep: 分隔符
        
    返回:
        拼接后的字符串
    """
    return sep.join(str(arg) for arg in args)

# 使用示例
print(smart_concat("Python", 3.9, "is", "great", sep="-"))
# 输出: Python-3.9-is-great

def build_multiline_text(lines):
    """
    构建多行文本
    
    参数:
        lines: 字符串列表
        
    返回:
        带行号的文本
    """
    return "\n".join(f"{i+1}. {line}" for i, line in enumerate(lines))

# 使用示例
text = build_multiline_text(["First line", "Second line", "Third line"])
print(text)
# 输出:
# 1. First line
# 2. Second line
# 3. Third line