import string
 
class FormatPlaceholder:
    def __init__(self, key):
        self.key = key
    def __format__(self, spec):
    	result = self.key
    	if spec:
    		result += ":" + spec
    	return "{" + result + "}"
 
class FormatDict(dict):
    def __missing__(self, key):
        return FormatPlaceholder(key)
 
s = '{foo} {bar} {baz:123xyz}'
formatter = string.Formatter()
mapping = FormatDict(foo='FOO')
print(formatter.vformat(s, (), mapping))

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