is there an easy way to parse a nested list ?

Stef Mientki stef.mientki at gmail.com
Mon Mar 16 21:49:26 CET 2009


thanks Aaron, Paul and Vlastimil,
sorry I phrased my question wrong,
I was looking for parsing an expression of an element of a nested list
the code below seems to do what I'm looking for.

cheers,
Stef


Aaron Brady wrote:
> On Mar 15, 6:44 pm, Stef Mientki <stef.mien... at gmail.com> wrote:
>   
>> hello,
>>
>> I need to parse a nested list, given as a string, like this
>>
>> line = " A  [  B  [ C+2 ] + 3 ] "
>>
>> ( I probably can do it with find, but I guess that's not the most
>> elegant way, besides recusrion is not my strongest point)
>>
>> which should be parsed so I get an list from inner to outer side (don't
>> know if these are the correct words),
>> but I should like to have a list or tuple for tis case:
>>
>> parsed = [ C+2,  B[C+2]+3,   A  [  B  [ C+2 ] + 3 ] ]
>>    or (last term is not needed, as well as the constants aren't)
>> parsed = [ C,  B[C+2] ]
>>
>> this all, to improve the improved error / exception message even more ;-)
>>
>> thanks,
>> Stef
>>     
>
> Hi Stef,
>
> This looks like what you want:
>
> def parse_list( s, start= 0 ):
>     x= []
>     i= start
>     while i< len( s ):
>         c= s[ i ]
>         if c== '[':
>             y, i= parse_list( s, i+ 1 )
>             x= x+ y
>         if c== ']':
>             return x+ [ s[start: i ] ], i
>         i+= 1
>     return x+ [ s ]
>
> line = " A  [  B  [ C+2 ] + 3 ] "
> print( parse_list( line ) )
> #[' C+2 ', '  B  [ C+2 ] + 3 ', ' A  [  B  [ C+2 ] + 3 ] ']
>
> line = " A  [  B  [ C+2 ] [ D+2 ] + 3 ] "
> print( parse_list( line ) )
> #[' C+2 ', ' D+2 ', '  B  [ C+2 ] [ D+2 ] + 3 ', ' A  [  B  [ C+2 ] [ D
> +2 ] + 3 ] ']
>
> ...So long as you don't have any brackets inside strings or what not.
> It just admits two special characters, '[' and ']'.
> --
> http://mail.python.org/mailman/listinfo/python-list
>   




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