Python 如何在字符串中插入变量

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

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

> 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.'
>

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

class safesub(dict):
"""防止key找不到"""
def __missing__(self, key):
  return '{' + key + '}'

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

import sys

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

> 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}
>

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

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