[Tutor] preventing KeyError from "%"

[Michael P. Reilly]

Lance E Sloan 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:
> 
> ^IKeyError: 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:
> 
> ^Ilance = {'first': 'Lance', 'mi': 'E', 'last': 'Sloan'}
> ^Imonty = {'first': 'Monty', 'last': 'Python'}
> 
> ^Iprint '%(first)s %(mi)s %(last)s\n' % lance
> ^Iprint '%(first)s %(mi)s %(last)s\n' % monty
> 
> it would produce:
> 
> ^ILance E Sloan
> ^IMonty  Python
> 
> instead of throwing an exception for the "% monty" line.

Sorry, but no.  All values must exist in the dictionary.
>From the Python Library Reference Manual:
   If the right argument is a dictionary (or any kind of mapping), then
   the formats in the string _must_ have a parenthesized key into that
   dictionary inserted immediately after the "%" character, and each
   format formats the corresponding entry from the mapping.
<URL: http://www.python.org/doc/current/lib/typesseq-strings.html>

You could make sure that all values are an empty string at first.
Create a dictionary with empty strings first, and populate it with
the values afterward.  Or go thru the dictionary and find the missing
fields and populate the dictionary that way.

  -Arcege

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| Michael P. Reilly                | arcege@speakeasy.net              |