Determining a replacement dictionary from scratch

[Dave Benjamin]

In article <4b805376.0401181116.3b0adb41 at posting.google.com>, Dan wrote:
> I'd like to be able to take a formatted string and determine the
> replacement dictionary necessary to do string interpolation with it. 
> For example:
> 
>>>> str = 'his name was %(name)s and i saw him %(years)s ago.'
>>>> createdict( str )
> {'name':'', 'years':''}
>>>>
> 
> Notice how it would automatically fill in default values based on
> type.  I figure since python does this automatically maybe there is a
> clever way to solve the problem.  Otherwise I will just have to parse
> the string myself.  Any clever solutions to this?

Here's a solution that uses a regular expression. It only handles strings
and doesn't cache the regular expression; I'll leave this up to you. =)

import re
def createdict(fmt):
    keys = re.findall('%\(([A-Za-z]+)\)s', fmt)
    return dict(zip(keys, [''] * len(keys)))
        
The regular expression works like this:

 %\(         - match a percent character and opening parenthesis
 ([A-Za-z]+) - match a sequence of one or more alpha characters as a GROUP
 \)s         - match a closing parenthesis and 's' character

For more details on "re", see the documentation:
http://python.org/doc/current/lib/module-re.html

HTH,
Dave

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