formatter.py

import string
class PartialFormatter(string.Formatter):
    def __init__(self, missing='~~', bad_fmt='!!'):
        self.missing, self.bad_fmt=missing, bad_fmt

    def get_field(self, field_name, args, kwargs):
        # Handle a key not found
        try:
            val=super(PartialFormatter, self).get_field(field_name, args, kwargs)
            # Python 3, 'super().get_field(field_name, args, kwargs)' works
        except (KeyError, AttributeError):
            val=None,field_name 
        return val 

    def format_field(self, value, spec):
        # handle an invalid format
        if value==None: return self.missing
        try:
            return super(PartialFormatter, self).format_field(value, spec)
        except ValueError:
            if self.bad_fmt is not None: return self.bad_fmt   
            else: raise