一、旧式的字符串格式化
% 操作符
参考以下示例：
> name = "Eric" > "Hello, %s." % name 'Hello, Eric.'
当有多个变量需要插入到字符串中时：
> name = "Eric" > age = 74 > "Hello, %s. You are %s." % (name, age) 'Hello, Eric. You are 74.'
当需要替换的变量进一步增多时，使用 % 操作符格式化字符串会导致代码可读性变得很差：
> first_name = "Eric" > last_name = "Idle" > age = 74 > profession = "comedian" > affiliation = "Monty Python" > "Hello, %s %s. You are %s. You are a %s. You were a member of %s." % (first_name, last_name, age, profession, affiliation) 'Hello, Eric Idle. You are 74. You are a comedian. You were a member of Monty Python.'
str.format()
str.format() 是对 % 方式的改进，它使用常见的函数调用的语法，并且可以通过定义对象本身的 __format__() 方法控制字符串格式化的具体行为。
基本用法：
> name = "Eric"
> age = 74
> "Hello, {}. You are {}.".format(name, age)
'Hello, Eric. You are 74.'
str.format() 相对于 % 操作符有着更强的灵活性。比如可以通过数字索引来关联替换到字符串中的变量：
> name = "Eric"
> age = 74
> "Hello, {1}. You are {0}.".format(age, name)
'Hello, Eric. You are 74.'
为了提高代码可读性，{} 中也可以使用有具体含义的参数名：
> name = "Eric"
> age = 74
> "Hello, {name}. You are {age}".format(name=name, age=age)
'Hello, Eric. You are 74'
针对字典结构的数据：
> person = {'name': 'Eric', 'age': 74}
> "Hello, {name}. You are {age}.".format(name=person['name'], age=person['age'])
'Hello, Eric. You are 74.'
或者更简洁的方式：
> person = {'name': 'Eric', 'age': 74}
> "Hello, {name}. You are {age}.".format(**person)
'Hello, Eric. You are 74.'
问题在于当需要替换的变量很多时，str.format() 方式依然会导致代码变得过于冗长：
> first_name = "Eric"
> last_name = "Idle"
> age = 74
> profession = "comedian"
> affiliation = "Monty Python"
> "Hello, {first_name} {last_name}. You are {age}. You are a {profession}. You were a member of {affiliation}." .format(first_name=first_name, last_name=last_name, age=age, profession=profession, affiliation=affiliation)
'Hello, Eric Idle. You are 74. You are a comedian. You were a member of Monty Python.'
二、f-string
基本用法
> name = "Eric"
> age = 74
> f"Hello, {name}. You are {age}."
'Hello, Eric. You are 74.'
嵌入表达式
> f"{2 * 37}"
'74'
> def to_lowercase(input):
... return input.lower()
> name = "Eric Idle"
> f"{to_lowercase(name)} is funny"
'eric idle is funny'
> f"{name.lower()} is funny"
'eric idle is funny'
f-string 中还可以直接嵌入某个对象实例，只要其内部实现了 __str__ 或者 __repr__ 方法：
class Comedian:
def __init__(self, first_name, last_name, age):
self.first_name = first_name
self.last_name = last_name
self.age = age
def __str__(self):
return f"{self.first_name} {self.last_name} is {self.age}"
new_comedian = Comedian("Eric", "Idle", 74)
print(f"{new_comedian}")
# Eric Idle is 74
多行 f-string
> name = "Eric"
> profession = "comedian"
> affiliation = "Monty Python"
> message = (
... f"Hi {name}. "
... f"You are a {profession}. "
... f"You were in {affiliation}."
... )
> message
'Hi Eric. You are a comedian. You were in Monty Python.'
参考资料
Python 3's f-Strings: An Improved String Formatting Syntax (Guide)
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