Leaving values blank if not passed in str.format

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

import stringclass 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}.

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.

For the record:

s = "My name is {name} and I'm really {adjective}."kwargs = dict((x[1], '') for x in s._formatter_parser())# Now we have: `kwargs = {'name':'', 'adjective':''}`.kwargs.update(name='mark')print s.format(**kwargs)  # My name is mark and I'm really .