Leaving values blank if not passed in str.format

Here is one option which uses collections.defaultdict:

>>> from collections import defaultdict
>>> kwargs = {"name": "mark"}
>>> template = "My name is {0[name]} and I'm really {0[adjective]}."
>>> template.format(defaultdict(str, kwargs))
"My name is mark and I'm really ."

Note that we aren't using ** to unpack the dictionary into keyword arguments anymore, and the format specifier uses {0[name]} and {0[adjective]}, which indicates that we should perform a key lookup on the first argument to format() using "name" and "adjective" respectively. By using defaultdict a missing key will result in an empty string instead of raising a KeyError.

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

import string

class BlankFormatter(string.Formatter):
    def __init__(self, default=''):
        self.default=default

    def get_value(self, key, args, kwds):
        if isinstance(key, str):
            return kwds.get(key, self.default)
        else:
            return string.Formatter.get_value(key, args, kwds)

kwargs = {"name": "mark", "adj": "mad"}     
fmt=BlankFormatter()
print fmt.format("My name is {name} and I'm really {adj}.", **kwargs)
# My name is mark and I'm really mad.
print fmt.format("My name is {name} and I'm really {adjective}.", **kwargs)
# My name is mark and I'm really .  

---

As of Python 3.2, you can use .format_map as an alternative:

class Default(dict):
    def __missing__(self, key):
        return '{'+key+'}'

kwargs = {"name": "mark"}

print("My name is {name} and I'm really {adjective}.".format_map(Default(kwargs)))

which prints:

My name is mark and I'm really {adjective}.