Check if a string to interpolate provides expected placeholders

Consider this fictitious Python function:

def f(s): # accepts a string containing placeholders # returns an interpolated string return s % {'foo': 'OK', 'bar': 'OK'}

How can I check that the string s provides all the expected placeholders, and if not, make the function politely show the missing keys?

My solution follows. My question: is there a better solution?

import sys def f(s): d = {} notfound = [] expected = ['foo', 'bar'] while True: try: s % d break except KeyError as e: key = e.args[0] # missing key notfound.append(key) d.update({key: None}) missing = set(expected).difference(set(notfound)) if missing: sys.exit("missing keys: %s" % ", ".join(list(missing))) return s % {'foo': 'OK', 'bar': 'OK'}


There's a way to see all of the named placeholders using the _formatter_parser method:

>>>> y="A %{foo} is a %{bar}" >>>> for a,b,c,d in y._formatter_parser(): print b foo bar

For a "public" way:

>>>> import string >>>> x = string.Formatter() >>>> elements = x.parse(y) >>>> for a,b,c,d in elements: print b


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