I was able to get the cast of the movie like this:
#!/usr/bin/env python
import imdb
ia = imdb.IMDb()
s_result = ia.search_movie('The Untouchables')
the_unt = s_result[0]
print the_unt['cast']
However, that gave all the cast, i am looking for just the stars of the movie. for example, al pacino is a star in god father
As stated in the comments, IMDb does not know the concept of a "star", but it does somewhat know the "top actors" from the cast lineup.
It is probably based on the credits of the film, but even IMDb says first billed only for some movies.
The cast ordering is entirely dependent on the movie and who verifies the data on IMDb. If it says "credits order", that means "in the order they were introduced in the film credits", but, the credit ordering could be some arbitrary ordering that the director of the film felt like placing them in.
For example, some films say "And introducing... (some actor no one knows about)" or like a TV Show says "Special Guest Star... (someone most people recognize)". In both cases, those are either before / after the entire regular cast is introduced.
So, if you wanted the top 5 actors for a given film, you could do something like this
import imdb
ia = imdb.IMDb()
search_results = ia.search_movie('The Godfather')
if search_results:
movieID = search_results[0].movieID
movie = ia.get_movie(movieID)
if movie:
cast = movie.get('cast')
topActors = 5
for actor in cast[:topActors]:
print "{0} as {1}".format(actor['name'], actor.currentRole)
Output
Marlon Brando as Don Vito Corleone
Al Pacino as Michael Corleone
James Caan as Sonny Corleone
Richard S. Castellano as Clemenza
Robert Duvall as Tom Hagen