Add custom conversion types for string formatting

You can create a custom formatter for html templates:

import string, cgi

class Template(string.Formatter):
    def format_field(self, value, spec):
        if spec.endswith('h'):
            value = cgi.escape(value)
            spec = spec[:-1] + 's'
        return super(Template, self).format_field(value, spec)

print Template().format('{0:h} {1:d}', "<hello>", 123)

Note that all conversion takes place inside the template class, no change of input data is required.

Not with % formatting, no, that is not expandable.

You can specify different formatting options when using the newer format string syntax defined for str.format() and format(). Custom types can implement a __format__() method, and that will be called with the format specification used in the template string:

import cgi

class HTMLEscapedString(unicode):
    def __format__(self, spec):
        value = unicode(self)
        if spec.endswith('h'):
            value = cgi.escape(value)
            spec = spec[:-1] + 's'
        return format(value, spec)

This does require that you use a custom type for your strings:

>>> title = HTMLEscapedString(u'Proof that 12 < 6')
>>> print "<title>{:h}</title>".format(title)
<title>Proof that 12 &lt; 6</title>

For most cases, it is easier just to format the string before handing it to the template, or use a dedicated HTML templating library such as Chameleon, Mako or Jinja2; these handle HTML escaping for you.