python 字符串格式化的示例

更新時間：2020年09月21日 09:40:51 作者：Starryland

這篇文章主要介紹了python 字符串格式化的示例，幫助大家更好的理解和使用python處理字符串，感興趣的朋友可以了解下

一、舊式的字符串格式化

% 操作符

參考以下示例：

>>> name = "Eric"
>>> "Hello, %s." % name
'Hello, Eric.'

當(dāng)有多個變量需要插入到字符串中時：

>>> name = "Eric"
>>> age = 74
>>> "Hello, %s. You are %s." % (name, age)
'Hello, Eric. You are 74.'

當(dāng)需要替換的變量進一步增多時，使用 % 操作符格式化字符串會導(dǎo)致代碼可讀性變得很差：

>>> 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() 是對 % 方式的改進，它使用常見的函數(shù)調(diào)用的語法，并且可以通過定義對象本身的 __format__() 方法控制字符串格式化的具體行為。

基本用法：

>>> name = "Eric"
>>> age = 74
>>> "Hello, {}. You are {}.".format(name, age)
'Hello, Eric. You are 74.'

str.format() 相對于 % 操作符有著更強的靈活性。比如可以通過數(shù)字索引來關(guān)聯(lián)替換到字符串中的變量：

>>> name = "Eric"
>>> age = 74
>>> "Hello, {1}. You are {0}.".format(age, name)
'Hello, Eric. You are 74.'

為了提高代碼可讀性，{} 中也可以使用有具體含義的參數(shù)名：

>>> name = "Eric"
>>> age = 74
>>> "Hello, {name}. You are {age}".format(name=name, age=age)
'Hello, Eric. You are 74'

針對字典結(jié)構(gòu)的數(shù)據(jù)：

>>> person = {'name': 'Eric', 'age': 74}
>>> "Hello, {name}. You are {age}.".format(name=person['name'], age=person['age'])
'Hello, Eric. You are 74.'

或者更簡潔的方式：

>>> person = {'name': 'Eric', 'age': 74}
>>> "Hello, {name}. You are {age}.".format(**person)
'Hello, Eric. You are 74.'

問題在于當(dāng)需要替換的變量很多時，str.format() 方式依然會導(dǎo)致代碼變得過于冗長：

>>> 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.'

嵌入表達式

>>> 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 中還可以直接嵌入某個對象實例，只要其內(nèi)部實現(xiàn)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.'

參考資料

Python 3's f-Strings: An Improved String Formatting Syntax (Guide)

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