Differences between Methods and functions

[adfgvx]

I have noticed a strange thing (for me). I have written a function that
recursively walk throught a tree produced by the pyRXP XML parser.
Everything is ok, fast and produces good result.

I transformed it a little (just add self. !) to include it in an object as a
method (still recursive) but now the walk is very much slower (about 100
times !!).
I don't understand why !

Any ideas ?

Thanks,

Bruno