partial string formatting

Question

Is it possible to do partial string formatting with the advanced string formatting methods, similar to the string template safe_substitute() function?

F

Answer 1

If you'd like to unpack a dictionary to pass arguments to format, as in this related question, you could use the following method.

First assume the string s is the same as in this question:

s = '{foo} {bar}'

and the values are given by the following dictionary:

replacements = {'foo': 'FOO'}

Clearly this won't work:

s.format(**replacements)
#---------------------------------------------------------------------------
#KeyError                                  Traceback (most recent call last)
# in ()
#----> 1 s.format(**replacements)
#
#KeyError: 'bar'

However, you could first get a set of all of the named arguments from s and create a dictionary that maps the argument to itself wrapped in curly braces:

from string import Formatter
args = {x[1]:'{'+x[1]+'}' for x in Formatter().parse(s)}
print(args)
#{'foo': '{foo}', 'bar': '{bar}'}

Now use the args dictionary to fill in the missing keys in replacements. For python 3.5+, you can do this in a single expression:

new_s = s.format(**{**args, **replacements}}
print(new_s)
#'FOO {bar}'

For older versions of python, you could call update:

args.update(replacements)
print(s.format(**args))
#'FOO {bar}'