wat.py

import string

class Template(string.Formatter):
    def format_field(self, value, spec):
        if spec.endswith('h'):
            value = str(value) # ultimately, this will call a funtion that will format the value and return as string
            spec = spec[:-1] + 's'
        return super(Template, self).format_field(value, spec)

f= Template()
print(f.format("Hello {:h}", "world"))