python中字符串格式化f-string和str.format有什么区别?

f-string和str.format()是Python中两种常用的字符串格式化方法，它们都可以通过大括号{}在字符串中嵌入变量或表达式的值。它们的区别主要有以下几点：

• f-string是Python 3.6引入的一种新的格式化方式，它以f或F为前缀，比如f"Hello, {name}."，而str.format()是Python 2.6引入的一种改进的格式化方式，它使用字符串对象的format()方法，比如"Hello, {name}.".format(name=name)。
• f-string的优点是更简洁、更易读、更快速，而且不容易出错。f-string在运行时计算大括号中的表达式的值，而str.format()需要先定义变量或字典，然后传递给format()方法。
• f-string可以直接访问局部变量和全局变量，而str.format()需要显式地传递变量名或使用**操作符进行字典拆包。f-string还可以在大括号中使用各种Python语法，比如三元运算符、列表推导、lambda函数等，而str.format()则不支持这些语法。
• f-string还可以使用格式描述符来控制字符串的对齐、填充、精度、类型等，比如f"{value:10.2f}"表示将value值格式化为10位宽度，保留两位小数的浮点数。str.format()也可以使用格式描述符，但需要在大括号中指定变量名或索引，比如"{value:10.2f}".format(value=value)。

# 使用f-string
name = "Eric"
age = 74
print(f"Hello, {name}. You are {age}.")
# 输出：Hello, Eric. You are 74.

# 使用str.format()
name = "Eric"
age = 74
print("Hello, {name}. You are {age}.".format(name=name, age=age))
# 输出：Hello, Eric. You are 74.

# 使用f-string访问局部变量和全局变量
def greet(name):
    greeting = "Hello"
    print(f"{greeting}, {name}. You are {age}.")
    
age = 74
greet("Eric")
# 输出：Hello, Eric. You are 74.

# 使用str.format()访问局部变量和全局变量
def greet(name):
    greeting = "Hello"
    print("{greeting}, {name}. You are {age}.".format(greeting=greeting, name=name, age=age))
    
age = 74
greet("Eric")
# 输出：Hello, Eric. You are 74.

# 使用f-string在大括号中使用Python语法
name = "Eric"
profession = "comedian"
affiliation = "Monty Python"
message = f"{name.upper()} is a {profession}. He was a member of {affiliation.lower()}." 
print(message)
# 输出：ERIC is a comedian. He was a member of monty python.

# 使用str.format()在大括号中使用Python语法
name = "Eric"
profession = "comedian"
affiliation = "Monty Python"
message = "{name.upper()} is a {profession}. He was a member of {affiliation.lower()}.".format(name=name, profession=profession, affiliation=affiliation) 
print(message)
# 输出：ERIC is a comedian. He was a member of monty python.

# 使用f-string使用格式描述符
value = 12.3456
print(f"The value is {value:10.2f}")
# 输出：The value is      12.35

# 使用str.format()使用格式描述符
value = 12.3456
print("The value is {value:10.2f}".format(value=value))
# 输出：The value is      12.35