python script to add a field and edit it for several shapefiles in arcgis

[yo]

dear All
i m trying to write a python script supposed to create a new field in
a shapefile, and then fill it with a value extracted from the name of
the file...
I have to do that for several hundreds of files...I am almost there it
is kind of working for one file but once i put a loop in the story,
then everything crashes...
could anybody here offer some help ???
regards
Yoann