Python 字符串格式化

name = "拾光"
age = 18

# 基本用法
print("Hello, %s!" % name)  # Hello, Alice!
print("%s is %d years old" % (name, age))

# 格式化选项
print("%10s" % name)      # 右对齐: '     拾光'
print("%-10s" % name)     # 左对齐: '拾光     '
print("%.2f" % 3.14159)   # 浮点数精度: '3.14'

# 字典格式化
print("%(name)s: %(age)d" % {"name": name, "age": age})

# 位置参数
print("{} is {} years old".format(name, age))

# 命名参数
print("{name} works as {job}".format(name="Bob", job="engineer"))

# 索引重用
print("{0} {1} {0}".format("a", "b"))  # a b a

# 格式化选项
print("{:.2f}".format(3.14159))      # 3.14
print("{:>10}".format(name))         # 右对齐: '     拾光'
print("{:<10}".format(name))         # 左对齐: '拾光     '
print("{:^10}".format(name))         # 居中: '  拾光   '
print("{:,}".format(1000000))        # 千分位: 1,000,000

# 类型转换
print("{!r}".format("text"))         # 调用repr(): 'text'

# 变量直接嵌入
print(f"{name} is {age} years old")

# 表达式计算
print(f"Next year {name} will be {age + 1}")

# 调用函数
print(f"Name uppercase: {name.upper()}")

# 多行f-string
message = (
    f"User: {name}\n"
    f"Age: {age}\n"
    f"Status: {'Adult' if age >= 18 else 'Minor'}"
)

# 数字格式化
pi = 3.1415926
print(f"Pi: {pi:.3f}")              # Pi: 3.142
print(f"Hex: {255:#x}")             # Hex: 0xff

# 对齐与填充
print(f"{name:>15}")                # 右对齐: '         拾光'
print(f"{name:*<10}")               # 左填充: '拾光*****'

# 日期格式化
from datetime import datetime
now = datetime.now()
print(f"Today: {now:%Y-%m-%d %H:%M}")  # Today: 2025-06-17 12:30

# 原始字符串 (处理特殊字符)
path = r"d:\folder"
print(fr"Path: {path}")             # Path: d:\folder

# 字典操作
user = {"name": "Alice", "age": 30}
print(f"{user['name']}: {user['age']}")

# 在format规范中嵌套变量
width = 10
precision = 3
print(f"{pi:{width}.{precision}f}") # '     3.142'

# 条件表达式
count = 5
print(f"You have {count} item{'s' if count != 1 else ''}")

# 复杂逻辑预处理
discount_rate = 0.15 if is_member else 0.05
total = price * quantity * (1 - discount_rate)
print(f"Final price: {total:.2f}")

# 危险！可能SQL注入
query = f"SELECT * FROM users WHERE name = '{name}'"

# 安全做法
query = "SELECT * FROM users WHERE name = %s"
cursor.execute(query, (name,))

print(f"{{}} = braces")  # {} = braces

num = 0.1 + 0.2
print(f"{num:.1f}")      # 0.3 (显示修正)
print(f"{num}")          # 0.30000000000000004

x = 42
print(f"{x = }")         # x = 42 (Python 3.8+)

value = None
print(f"Value: {value or 'N/A'}")  # Value: N/A

big_num = 10**9
print(f"{big_num:,}")    # 1,000,000,000
print(f"{big_num:_}")    # 1_000_000_000