Modern Python Standard Library Cookbook

How to do it...

The steps for this recipe are:

1. The string.Formatter object allows you to extend its syntax, so we can specialize it to support injecting code into the expressions that it's going to accept:

import string

class TemplateFormatter(string.Formatter):
    def get_field(self, field_name, args, kwargs):
        if field_name.startswith("$"):
            code = field_name[1:]
            val = eval(code, {}, dict(kwargs))
            return val, field_name
        else:
            return super(TemplateFormatter, self).get_field(field_name, args, kwargs)

2. Our TemplateFormatter can then be used to generate text similar to our example in a much cleaner way:

messages = ['Message 1', 'Message 2'] tmpl = TemplateFormatter() txt = tmpl.format("Hello {name}, " "You have {$len(messages)} message{$len(messages) ...