Python中Template使用的一个小技巧

Python中Template是string中的一个类，可以将字符串的格式固定下来，重复利用。

from string import Template
s = Template("there are ${howmany} ${lang} Quotation symbols")
print s.substitute(lang='Python',howmany=3)
>>>there are 3  Python Quotation symbols

用法很简单，先生成一个模板实例s,然后调用替换函数substitute()将模板中的两个地方替换掉。替换的内容是通过字典对调用的，所以下面(lang='Python',howmany=3)出现的顺序可以不用严格的和模板中的一样。当然不用括号也是可以的。

from string import Template
s = Template("there are $howmany $lang Quotation symbols")
print s.substitute(lang='Python',howmany=3)
>>>there are 3  Python Quotation symbols

注意：在使用${howmany} ${lang}时候，括号里的内容和括号要紧贴着，不然会报错。

from string import Template
s = Template("there are ${ howmany } ${lang} Quotation symbols")
print s.substitute(lang='Python',howmany=3)
>>>Traceback (most recent call last):
  File "E:/�������/201703/DeepLearning/neural-networks-and-deep-learning-master/src/validation.py", line 39, in <module>
    print s.substitute(lang='Python',howmany=3)
  File "C:\Users\wangxin\Anaconda2\lib\string.py", line 176, in substitute
    return self.pattern.sub(convert, self.template)
  File "C:\Users\wangxin\Anaconda2\lib\string.py", line 173, in convert
    self._invalid(mo)
  File "C:\Users\wangxin\Anaconda2\lib\string.py", line 146, in _invalid
    (lineno, colno))
ValueError: Invalid placeholder in string: line 1, col 11

当然在使用substitute()的时候，对应的关键字和值都要给出，不然会报错。

from string import Template
s = Template("there are ${ howmany } ${lang} Quotation symbols")
print s.substitute(lang='Python')
>>>Traceback (most recent call last):
  File "E:/�������/201703/DeepLearning/neural-networks-and-deep-learning-master/src/validation.py", line 39, in <module>
    print s.substitute(lang='Python')
  File "C:\Users\wangxin\Anaconda2\lib\string.py", line 176, in substitute
    return self.pattern.sub(convert, self.template)
  File "C:\Users\wangxin\Anaconda2\lib\string.py", line 166, in convert
    val = mapping[named]
KeyError: 'howmany'

使用safe_substitute()可以避免报错.

from string import Template
s = Template("there are ${howmany} ${lang} Quotation symbols")
print s.safe_substitute(lang='Python')
>>>there are ${howmany} Python Quotation symbols