python - Add custom conversion types for string formatting

Question

Is there anyway in python to add additional conversion types to string formatting?

The standard conversion types used in %-based string formatting are things like s for strings, d for decimals, etc. What I'd like to do is add a new character for which I can specify a custom handler (for instance a lambda function) that will return the string to insert.

For instance, I'd like to add h as a conversion type to specify that the string should be escaped for using in HTML. As an example:

#!/usr/bin/python

print "<title>%(TITLE)h</title>" % {"TITLE": "Proof that 12 < 6"}

And this would use cgi.escape on the "TITLE" to produce the following output:

<title>Proof that 12 &lt; 6</title>

Answer 1

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.