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

My suggestion would be the following (tested with Python3.6):

class Lazymap(object):
       def __init__(self, **kwargs):
           self.dict = kwargs

       def __getitem__(self, key):
           return self.dict.get(key, "".join(["{", key, "}"]))


s = '{foo} {bar}'

s.format_map(Lazymap(bar="FOO"))
# >>> '{foo} FOO'

s.format_map(Lazymap(bar="BAR"))
# >>> '{foo} BAR'

s.format_map(Lazymap(bar="BAR", foo="FOO", baz="BAZ"))
# >>> 'FOO BAR'

Update: An even more elegant way (subclassing dict and overloading __missing__(self, key)) is shown here: https://stackoverflow.com/a/17215533/333403