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How to escape $ on Python string Template class?

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无人共我
无人共我 2021-01-01 17:56

Introduction

The string module has a Template class, that lets you make substitutions in a string using a mapping object, for instance:

>>>
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  • 走了就别回头了
    2021-01-01 18:19

    Python will not do string substitution over multiple lines

    If you have this string

    criterion = """
    <criteria>
    <order>{order}</order>
      <body><![CDATA[{code}]]></body>
    </criteria>
"""

criterion.format(dict(order="1",code="Hello")

    results in: 

    KeyError: 'order'

    A solution is to use the string.Template module

    from string import Template

criterion = """
    <criteria>
    <order>$order</order>
      <body><![CDATA[$code]]></body>
    </criteria>
"""

Template(criterion).substitute(dict(order="1",code="hello")

    NOTE: you have to prefix the keywords with a $ not wrap them in {}

    output is: 

     <criteria>
    <order>1</order>
      <body><![CDATA[hello]]></body>
    </criteria>

    Full docs are: https://docs.python.org/2/library/string.html#template-strings

    0 讨论(0)
  • 小鲜肉
    2021-01-01 18:27

    The docs say that you can replace the pattern as long as it contains all necessary named groups:

    import re
from string import Template


class TemplateIgnoreInvalid(Template):
    # override pattern to make sure `invalid` never matches
    pattern = r"""
    %(delim)s(?:
      (?P<escaped>%(delim)s) |   # Escape sequence of two delimiters
      (?P<named>%(id)s)      |   # delimiter and a Python identifier
      {(?P<braced>%(id)s)}   |   # delimiter and a braced identifier
      (?P<invalid>^$)            # never matches (the regex is not multilined)
    )
    """ % dict(delim=re.escape(Template.delimiter), id=Template.idpattern)


def check_substitution(template, **mapping):
    try:
        TemplateIgnoreInvalid(template).substitute(mapping)
    except KeyError:
        return False
    else:
        return True

    Tests

    f = check_substitution
assert f('var is $var', var=1)
assert f('$ var is $var', var=1)
assert     f('var is $var and foo is $foo', var=1, foo=2)
assert not f('var is $var and foo is $foo', var=1)
assert     f('$ var is $var and foo is $foo', var=1, foo=2)
assert not f('$ var is $var and foo is $foo', var=1)
# support all invalid patterns
assert f('var is $var and foo is ${foo', var=1)
assert f('var is $var and foo is ${foo', var=1, foo=2) #NOTE: problematic API
assert     f('var is $var and foo is ${foo and ${baz}', var=1, baz=3)
assert not f('var is $var and foo is ${foo and ${baz}', var=1)

    It works for all invalid occurences of the delimiter ($).

    The examples show that ignoring invalid patterns conceals simple typos in the template so it is not a good API.

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  • 余生分开走
    2021-01-01 18:36

    This is a Quick Fix (Using recursion):

    def check_substitution(tem, m):
    try:
        string.Template(tem).substitute(m)
    except KeyError:
        return False
    except ValueError:
        return check_substitution(tem.replace('$ ', '$'), m) #strip spaces after $
    return True

    I Know its take a longer time if there is more than One Space between $ and var , so you may improve it by using Regular Expression.

    EDIT

    escaping $ into $$ makes more sense [ Thanks @Pedro ] so you can catch ValueError by this statement:

    return check_substitution(tem.replace('$ ', '$$ '), m) #escaping $ by $$
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