Python的string模块中的Template类字符串模板用法

string.Template()
string.Template()内添加替换的字符, 使用"$"符号, 或 在字符串内, 使用"${}"; 调用时使用string.substitute(dict)函数.
可以通过继承"string.Template", 覆盖变量delimiter(定界符)和idpattern(替换格式), 定制不同形式的模板.

代码:

    # -*- coding: utf-8 -*- 

    import string 

    template_text = ''''' 
      Delimiter : %% 
      Replaced : %with_underscore 
      Ingored : %notunderscored 
    ''' 

    d = {'with_underscore' : 'replaced', 
       'notunderscored' : 'not replaced'} 

    class MyTemplate(string.Template): 
      delimiter = '%' 
      idpattern = '[a-z]+_[a-z]+' 

    t = MyTemplate(template_text) 
    print('Modified ID pattern: ') 
    print(t.safe_substitute(d)) 

输出:

    Modified ID pattern:  

      Delimiter : % 
      Replaced : replaced 
      Ingored : %notunderscored 

注意: 定界符(delimiter)为"%", 替换模式(idpattern)必须包含下划线, 所以第2个没有进行替换.

正则替换

string.Template的pattern是一个正则表达式, 可以通过覆盖pattern属性, 定义新的正则表达式.
如: 使用新的定界符"{{", 把{{var}}作为变量语法.

代码:

    import string 

    t = string.Template('$var') 
    print(t.pattern.pattern) 

    class MyTemplate(string.Template): 
      delimiter = '{{' 
      pattern = r''''' 
      \{\{(?: 
       (?P<escaped>\{\{) |  # Escape sequence of two delimiters 
       (?P<named>[_a-z][_a-z0-9]*)\}\}   |  # delimiter and a Python identifier 
       {(?P<braced>[_a-z][_a-z0-9]*)}\}\}  |  # delimiter and a braced identifier 
       (?P<invalid>)       # Other ill-formed delimiter exprs 
      ) 
      ''' 

    t2 = MyTemplate(''''' 
    {{{{ 
    {{var}} 
    ''') 

    print('MATCHES: ', t2.pattern.findall(t2.template)) 
    print('SUBSTITUTED: ', t2.safe_substitute(var='replacement')) 

输出:

      \$(?: 
       (?P<escaped>\$) |  # Escape sequence of two delimiters 
       (?P<named>[_a-z][_a-z0-9]*)   |  # delimiter and a Python identifier 
       {(?P<braced>[_a-z][_a-z0-9]*)}  |  # delimiter and a braced identifier 
       (?P<invalid>)       # Other ill-formed delimiter exprs 
      ) 

    MATCHES: [('{{', '', '', ''), ('', 'var', '', '')] 
    SUBSTITUTED:  
    {{ 
    replacement 

字符串模板的安全替换(safe_substitute)
字符串模板(sting.Template), 替换时, 使用substitute(), 未能提供模板所需的全部参数值时, 会发生异常.
如果使用safe_substitute(), 即安全替换, 则会替换存在的字典值, 保留未存在的替换符号.

代码:

    import string 

    values = {'var' : 'foo'} 

    t = string.Template('''''$var is here but $ missing is not provided! ''') 


    try: 
      print 'substitute() : ', t.substitute(values) 
    except ValueError as err: 
      print 'Error:', str(err) 

    print 'safe_substitude() : ', t.safe_substitute(values) 

输出:

    substitute() : Error: Invalid placeholder in string: line 1, col 18 
    safe_substitude() : foo is here but $ missing is not provided!