You can follow the recommendation in PEP 3101 and subclass Formatter:

from __future__ import print_function
import string

class MyFormatter(string.Formatter):
    def __init__(self, default='{{{0}}}'):
        self.default=default
    
    def get_value(self, key, args, kwds):
        if isinstance(key, str):
            return kwds.get(key, self.default.format(key))
        else:
            Formatter.get_value(key, args, kwds)

Now try it:

>>> fmt=MyFormatter()
>>> fmt.format("{bond}, {james} {bond}", bond='bond', james='james')
'bond, james bond'
>>> fmt.format("{bond}, {james} {bond}", bond='bond')
'bond, {james} bond'

You can change how key errors are flagged by changing the text in self.default to what you would like to show for KeyErrors:

>>> fmt=MyFormatter('">>{{{0}}} KeyError<<"')
>>> fmt.format("{bond}, {james} {bond}", bond='bond', james='james')
'bond, james bond'
>>> fmt.format("{bond}, {james} {bond}", bond='bond')
'bond, ">>{james} KeyError<<" bond'

The code works unchanged on Python 2.6, 2.7, and 3.0+

You can follow the recommendation in PEP 3101 and subclass Formatter:

from __future__ import print_function
import string

class MyFormatter(string.Formatter):
    def __init__(self, default='{{{0}}}'):
        self.default=default
    
    def get_value(self, key, args, kwds):
        if isinstance(key, str):
            return kwds.get(key, self.default.format(key))
        else:
            return string.Formatter.get_value(key, args, kwds)

Now try it:

>>> fmt=MyFormatter()
>>> fmt.format("{bond}, {james} {bond}", bond='bond', james='james')
'bond, james bond'
>>> fmt.format("{bond}, {james} {bond}", bond='bond')
'bond, {james} bond'

You can change how key errors are flagged by changing the text in self.default to what you would like to show for KeyErrors:

>>> fmt=MyFormatter('">>{{{0}}} KeyError<<"')
>>> fmt.format("{bond}, {james} {bond}", bond='bond', james='james')
'bond, james bond'
>>> fmt.format("{bond}, {james} {bond}", bond='bond')
'bond, ">>{james} KeyError<<" bond'

The code works unchanged on Python 2.6, 2.7, and 3.0+