Notes on Python cookbook 3rd (2.15): inserting variables into strings

Giant ship 2020-11-14 01:37:23
notes python cookbook 3rd rd

Insert variable into string


You want to create a string with embedded variables , The variable is replaced by the string represented by its value .


Python There is no direct support for simply replacing variable values in strings . But by using string format() To solve this problem . such as :

>>> s = '{name} has {n} messages.'
>>> s.format(name='Guido', n=37)
'Guido has 37 messages.'

perhaps , If the variable to be replaced can be found in the variable field , So you can use it in combination format map() and vars() . It looks like this :

>>> name = 'Guido'
>>> n = 37
>>> s.format_map(vars())
'Guido has 37 messages.'

vars() Another interesting feature is that it also applies to object instances . such as :

>>> class Info:
... def __init__(self, name, n):
... = name
... self.n = n
>>> a = Info('Guido',37)
>>> s.format_map(vars(a))
'Guido has 37 messages.'

format and format map() One of the drawbacks of the is that they don't handle missing variables very well , such as :

>>> s.format(name='Guido')
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
KeyError: 'n'

One way to avoid this mistake is to define another that contains __missing__() Method's dictionary object , It looks like this :

class safesub(dict):
""" prevent key Can't find """
def __missing__(self, key):
return '{' + key + '}'

Now you can use this class to wrap input and pass it to format_map() :

>>> del n # Make sure n is undefined
>>> s.format_map(safesub(vars()))
'Guido has {n} messages.'

If you find yourself performing these steps frequently in your code , You can encapsulate the variable substitution step with a utility function . It looks like this :

import sys
def sub(text):
return text.format_map(safesub(sys._getframe(1).f_locals))

Now you can write like this :

>>> name = 'Guido'
>>> n = 37
>>> print(sub('Hello {name}'))
Hello Guido
>>> print(sub('You have {n} messages.'))
You have 37 messages.
>>> print(sub('Your favorite color is {color}'))
Your favorite color is {color}


Over the years, due to Python The lack of built-in support for variable substitution leads to various solutions . As one of the possible solutions shown in this section , You can sometimes see string formatting code like this :

>>> name = 'Guido'
>>> n = 37
>>> '%(name) has %(n) messages.' % vars()
'Guido has 37 messages.'

You may also see the use of string templates :

>>> import string
>>> s = string.Template('$name has $n messages.')
>>> s.substitute(vars())
'Guido has 37 messages.'

However , format() and format map() Compared with the above plans, it's just More advanced , So it should be preferred . Use format() Another advantage of the method is that you can get all the support for string formatting ( alignment , fill , Number format wait ), And these features are not possible with schemes like Template Strings .

Little known in mapping or dictionary classes __missing__() Method allows you to define how to handle missing values . stay SafeSub Class , This method is defined as returning a placeholder for the missing value . You can see that missing values appear in the result string ( It can be useful when debugging ), Instead of producing a KeyError abnormal .

sub() Function USES sys. getframe(1) Returns the caller's stack frame . You can access properties from f_locals To get local variables . There is no doubt that in most cases, it is not recommended to operate stack frames directly in code . however , It's very useful for functions like string replacement tools . in addition , It is worth noting that f_locals Is a dictionary that copies the local variables of the calling function . Although you can change f_locals The content of , But this change has no effect on subsequent variable access . therefore , Although accessing a stack frame seems evil , But any operation on it does not override or change the value of the caller's local variable .

本文为[Giant ship]所创,转载请带上原文链接,感谢

  1. 利用Python爬虫获取招聘网站职位信息
  2. Using Python crawler to obtain job information of recruitment website
  3. Several highly rated Python libraries arrow, jsonpath, psutil and tenacity are recommended
  4. Python装饰器
  5. Python实现LDAP认证
  6. Python decorator
  7. Implementing LDAP authentication with Python
  8. Vscode configures Python development environment!
  9. In Python, how dare you say you can't log module? ️
  10. 我收藏的有关Python的电子书和资料
  11. python 中 lambda的一些tips
  12. python中字典的一些tips
  13. python 用生成器生成斐波那契数列
  14. python脚本转pyc踩了个坑。。。
  15. My collection of e-books and materials about Python
  16. Some tips of lambda in Python
  17. Some tips of dictionary in Python
  18. Using Python generator to generate Fibonacci sequence
  19. The conversion of Python script to PyC stepped on a pit...
  20. Python游戏开发,pygame模块,Python实现扫雷小游戏
  21. Python game development, pyGame module, python implementation of minesweeping games
  22. Python实用工具,email模块,Python实现邮件远程控制自己电脑
  23. Python utility, email module, python realizes mail remote control of its own computer
  24. 毫无头绪的自学Python,你可能连门槛都摸不到!【最佳学习路线】
  25. Python读取二进制文件代码方法解析
  26. Python字典的实现原理
  27. Without a clue, you may not even touch the threshold【 Best learning route]
  28. Parsing method of Python reading binary file code
  29. Implementation principle of Python dictionary
  30. You must know the function of pandas to parse JSON data - JSON_ normalize()
  31. Python实用案例,私人定制,Python自动化生成爱豆专属2021日历
  32. Python practical case, private customization, python automatic generation of Adu exclusive 2021 calendar
  33. 《Python实例》震惊了,用Python这么简单实现了聊天系统的脏话,广告检测
  34. "Python instance" was shocked and realized the dirty words and advertisement detection of the chat system in Python
  35. Convolutional neural network processing sequence for Python deep learning
  36. Python data structure and algorithm (1) -- enum type enum
  37. 超全大厂算法岗百问百答(推荐系统/机器学习/深度学习/C++/Spark/python)
  38. 【Python进阶】你真的明白NumPy中的ndarray吗?
  39. All questions and answers for algorithm posts of super large factories (recommended system / machine learning / deep learning / C + + / spark / Python)
  40. [advanced Python] do you really understand ndarray in numpy?
  41. 【Python进阶】Python进阶专栏栏主自述:不忘初心,砥砺前行
  42. [advanced Python] Python advanced column main readme: never forget the original intention and forge ahead
  43. python垃圾回收和缓存管理
  44. java调用Python程序
  45. java调用Python程序
  46. Python常用函数有哪些?Python基础入门课程
  47. Python garbage collection and cache management
  48. Java calling Python program
  49. Java calling Python program
  50. What functions are commonly used in Python? Introduction to Python Basics
  51. Python basic knowledge
  52. Anaconda5.2 安装 Python 库(MySQLdb)的方法
  53. Python实现对脑电数据情绪分析
  54. Anaconda 5.2 method of installing Python Library (mysqldb)
  55. Python implements emotion analysis of EEG data
  56. Master some advanced usage of Python in 30 seconds, which makes others envy it
  57. python爬取百度图片并对图片做一系列处理
  58. Python crawls Baidu pictures and does a series of processing on them
  59. python链接mysql数据库
  60. Python link MySQL database