"".format用法

在Python中，`str.format()` 方法提供了一种格式化字符串的方法。它允许你通过大括号 `{}` 占位符将变量传递到字符串中，并可以控制变量的显示方式。以下是几种常见的用法：

1. 使用位置参数格式化字符串：

name = "World"
greeting = "Hello, {}!".format(name)
print(greeting)  # 输出: Hello, World!

2. 使用关键字参数：

greeting = "Hello, {name}! You are {age} years old.".format(name="Alice", age=25)
print(greeting)  # 输出: Hello, Alice! You are 25 years old.

3. 访问对象的属性或字典的键：

class Person:
    def __init__(self, first_name, last_name):
        self.first_name = first_name
        self.last_name = last_name

person = Person("John", "Doe")
greeting = "Hello, {0.first_name} {0.last_name}!".format(person)
print(greeting)  # 输出: Hello, John Doe!

4. 使用数字指定参数位置：

greeting = "Hello, {1} {0}!".format("World", "Alice")
print(greeting)  # 输出: Hello, Alice World!

5. 使用 `:` 进行填充、对齐、宽度和精度控制：

number = 123456.789
# 填充和对齐
formatted_number = "{:*>10}".format(number)  # 输出: *****123456.8
formatted_number = "{:*>10.2f}".format(number)  # 输出: *****123456.79

# 宽度和精度
formatted_number = "{:8}".format(number)  # 输出:   123456.8
formatted_number = "{:08.2f}".format(number)  # 输出: 0123456.79

6. 使用 `+`, `-`, `=` 进行正负号控制：

number = -123456.789
formatted_number = "{:>+10.2f}".format(number)  # 输出: -    123456.79
formatted_number = "{:>+10.2f}".format(-number)  # 输出: +    123456.79
formatted_number = "{:=+10.2f}".format(number)  # 输出: +123456.79