Lookup escape filter method

Question

Mako's Filtering and Buffering talks all about filters and how to use them. But is it possible to access the method used by a filter itself?

I want to properly escape a value in an attribute that will conditionally be rendered. Essentially I want to do this:

<element ${'attribute="${value | h}"' if value else ''}>

Which is obviously invalid because you can't nest expressions in strings. So I tried:

<element ${'attribute="{}"'.format(h(value)) if value else ''}>

However, this fails with a NameError: 'h' is not defined. I would like to avoid doing:

<element
    % if value:
        attribute="${value | h}"
    % endif
    >

Or:

<element ${'attribute="' if value else ''}${value or '' | h}${'"' if value else ''}>

I know I could use one of the multiple different methods available for escaping HTML attributes: cgi.escape(value, True), xml.sax.saxutils.quoteattr(value) (except it determines the quotes for you), or markupsafe.escape(value) (third-party). I would prefer to use the method used by Mako if at all possible. Is there a way to access Mako's build-in h using a lookup facility of some sort?

Answer 1

Short answer

The h escape filter is mapped to mako.filters.html_escape(). E.g.,

from mako.filters import html_escape as h

Long answer

The escape filters are mapped to the dotted names of functions in mako.filters.DEFAULT_ESCAPES:

DEFAULT_ESCAPES = {
    'x': 'filters.xml_escape',
    'h': 'filters.html_escape',
    'u': 'filters.url_escape',
    'trim': 'filters.trim',
    'entity': 'filters.html_entities_escape',
    'unicode': 'unicode', # str in python 3
    'decode': 'decode',
    'str': 'str',
    'n': 'n'
}

The n and decode filters are special conditions and do not map to a specific function. The others are either built-in functions or functions from mako.filters. These can be evaluated in 2 ways:

1. The potentially dangerous way:

   def lookup_escape(name):
       from mako import filters
       return eval(filters.DEFAULT_ESCAPES[name])
   

2. The safer way:

   import functools
   try:
       import builtins
   except ImportError:
       import __builtin__ as builtins
   import mako.filters
   
   def lookup_escape(name):
       path_parts = mako.filters.DEFAULT_ESCAPES[name].split('.')
       for module in (mako, builtins):
           try:
               return functools.reduce(getattr, path_parts, module)
           except AttributeError:
               pass
       raise LookupError(name)
   

Finally, the desired escape filter can be gotten with:

h = lookup_escape('h')