String format with optional dict key-value

Use defaultdict, this will allow you to specify a default value for keys which don't exist in the dictionary. For example:

>>> from collections import defaultdict
>>> d = defaultdict(lambda: 'UNKNOWN')
>>> d.update({'greetings': 'hello'})
>>> '%(greetings)s  %(name)s !!!' % d
'hello  UNKNOWN !!!'
>>> 

Some alternates to defaultDict,

greeting_dict = {'greetings': 'hello'}

if 'name' in greeting_dict :
    opening_line = '{greetings} {name}'.format(**greeting_dict)
else:
    opening_line = '{greetings}'.format(**greeting_dict)

print opening_line

Maybe even more succinctly, use dictionary get to set per parameter defaults,

'{greetings} {name}'.format(greetings=greeting_dict.get('greetings','hi'),
                            name=greeting_dict.get('name',''))