Default kwarg values for Python's str.format() method

Question

I'm looking to try and keep pluralisation of existing strings as simple as possible, and was wondering if it was possible to get str.format() to interpret a default value when looking for kwargs. Here's an example:

string = "{number_of_sheep} sheep {has} run away"
dict_compiled_somewhere_else = {'number_of_sheep' : 4, 'has' : 'have'}

string.format(**dict_compiled_somewhere_else)
# gives "4 sheep have run away"

other_dict = {'number_of_sheep' : 1}
string.format(**other_dict)
# gives a key error: u'has'
# What I'd like is for format to somehow default to the key, or perhaps have some way of defining the 'default' value for the 'has' key 
# I'd have liked: "1 sheep has run away"

Cheers

Answer 1

As PEP 3101, string.format(**other_dict) is not available.

If the index or keyword refers to an item that does not exist, then an IndexError/KeyError should be raised.

A hint for solving the problem is in Customizing Formatters, PEP 3101. That uses string.Formatter.

I improve the example in PEP 3101:

from string import Formatter

class UnseenFormatter(Formatter):
    def get_value(self, key, args, kwds):
        if isinstance(key, str):
            try:
                return kwds[key]
            except KeyError:
                return key
        else:
            return Formatter.get_value(key, args, kwds)

string = "{number_of_sheep} sheep {has} run away"
other_dict = {'number_of_sheep' : 1}

fmt = UnseenFormatter()
print fmt.format(string, **other_dict)

The output is

1 sheep has run away