Leaving values blank if not passed in str.format

Question

I've run into a fairly simple issue that I can't come up with an elegant solution for.

I'm creating a string using str.format in a function that is passed in a dict of substitutions to use for the format. I want to create the string and format it with the values if they're passed and leave them blank otherwise.

Ex

kwargs = {"name": "mark"} "My name is {name} and I'm really {adjective}.".format(**kwargs) 

should return

"My name is mark and I'm really ." 

instead of throwing a KeyError (Which is what would happen if we don't do anything).

Embarrassingly, I can't even come up with an inelegant solution for this problem. I guess I could solve this by just not using str.format, but I'd rather use the built-in (which mostly does what I want) if possible.

Note: I don't know in advance what keys will be used. I'm trying to fail gracefully if someone includes a key but doesn't put it in the kwargs dict. If I knew with 100% accuracy what keys would be looked up, I'd just populate all of them and be done with it.

Answer 1

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}. 

Answer 2

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.