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

problem

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

solution

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):
... self.name = 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}
>>>

Discuss

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 .

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