Creating formatted output using picture strings

[Alf P. Steinbach]

* Olof Bjarnason:
> 2010/2/10 Peter Otten <__peter__ at web.de>:
>> python at bdurham.com wrote:
>>
>>> Does Python provide a way to format a string according to a
>>> 'picture' format?
>>>
>>> For example, if I have a string '123456789' and want it formatted
>>> like '(123)-45-(678)[9]', is there a module or function that will
>>> allow me to do this or do I need to code this type of
>>> transformation myself?
>> A basic implementation without regular expressions:
>>
>>>>> def picture(s, pic, placeholder="@"):
>> ...     parts = pic.split(placeholder)
>> ...     result = [None]*(len(parts)+len(s))
>> ...     result[::2] = parts
>> ...     result[1::2] = s
>> ...     return "".join(result)
>> ...
>>>>> picture("123456789", "(@@@)-@@-(@@@)[@]")
>> '(123)-45-(678)[9]'
>>
>> Peter
>> --
>> http://mail.python.org/mailman/listinfo/python-list
>>
> 
> Inspired by your answer here's another version:
> 
>>>> def picture(s, pic):
> ...   if len(s)==0: return pic
> ...   if pic[0]=='#': return s[0]+picture(s[1:], pic[1:])
> ...   return pic[0]+picture(s, pic[1:])
> ...
>>>> picture("123456789", "(###)-##-(###)[#]")
> '(123)-45-(678)[9]'

I learned a bit by Peter Otten's example; I would have gotten to that notation 
sooner or later, but that example made it 'sooner' :-).

I think your version is cute.

I'd probably write it in a non-recursive way, though, like

     def picture( s, pic, placeholder = "@" ):
         result = ""
         char_iter = iter( s )
         for c in pic:
             result += c if c != placeholder else next( char_iter )
         return result

Of course this is mostly personal preference, but there is also a functional 
difference.

With your version an IndexError will be raised if there are too /many/ 
characters in s, while too few characters in s will yield "#" in the result.

With my version a StopIteration will be raised if there are to /few/ characters 
in s, while too many characters will just have the extraneous chars ignored.


Cheers,

- Alf