Python的string模块中的Template类字符串模板用法

转自：http://www.thinksaas.cn/topics/0/590/590916.html

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!