[Tutor] preventing KeyError from "%"

[Remco Gerlich]

On  0, Lance E Sloan <lsloan@umich.edu> wrote:
> I use the "%" operator with strings a lot.  One of the things I
> commonly do is set a dictionary from values submitted via a web page
> form or read from a database.  It's not uncommon that some fields may
> be empty and there wouldn't be a key for that field in the dictionary.
> When I use that dictionary with "%" and the format string calls for
> that key, I get this exception:
> 
> 	KeyError: x
> 
> How can I get "%" to not throw an exception, but instead skip over that
> key and move on to the next substitution?  For example, if I have
> this:
> 
> 	lance = {'first': 'Lance', 'mi': 'E', 'last': 'Sloan'}
> 	monty = {'first': 'Monty', 'last': 'Python'}
> 
> 	print '%(first)s %(mi)s %(last)s\n' % lance
> 	print '%(first)s %(mi)s %(last)s\n' % monty
> 
> it would produce:
> 
> 	Lance E Sloan
> 	Monty  Python
> 
> instead of throwing an exception for the "% monty" line.

You can't do that with the standard dictionary, so you have to roll your
own, inherited from UserDict, i.e.:

from UserDict import UserDict

class MyDict(UserDict):
   def __getitem__(self, arg):
      # Returns "" for any arg that's not in the underlying dictionary
      if self.data.has_key(arg):
         return self.data[arg]
      else:
         return ""

print '%(first)s %(mi)s %(last)s\n' % MyDict(lance)

-- 
Remco Gerlich