ignoring some placeholders in string formatting

[MRAB]

Michal Ludvig wrote:
> Hi all,
> 
> when I've got a string, say:
> 
> URL="http://xyz/blah?session=%(session)s&message=%(message)s"
> 
> is it possible to fill in only 'session' and leave "%(message)s" as is
> when it isn't present in the values dict?
> 
> For example:
> URL % { 'session' : 123 }
> raises KeyError because of missing 'message' in the dict.
> 
> I could indeed replace '%(session)s' with a string replace or regexp but
> that's not very elegant ;-)
> 
> Is there any way to tell the formatter to use only what's available and
> ignore the rest?
> 
You could write a class inheriting from dict which, for example, returns
"%(key)s" if the key "key" is absent:

 >>> class IgnoreDict(dict):
	def __getitem__(self, key):
		try:
			return dict.__getitem__(self, key)
		except KeyError:
			return "%%(%s)s" % key

		
 >>> d = {'session': 123}
 >>> URL = "http://xyz/blah?session=%(session)s&message=%(message)s"
 >>> URL % d

Traceback (most recent call last):
   File "<pyshell#4>", line 1, in <module>
     URL % d
KeyError: 'message'
 >>> URL % IgnoreDict(d)
'http://xyz/blah?session=123&message=%(message)s'