python_grouping.py

from itertools import groupby
from operator import itemgetter

myvals = [('hello', 'arrival'), ('hi', 'arrival'), ('test', 'no'), ('voo', 'yes'), ('cool', 'arrival')]

[(a,map(itemgetter(1), b)) 
      for a,b in groupby(                                                # 4. Group by key for which you have create list.
        sorted([(x,                                                      # 3. Sort the keys like arrival, yes, etc.
        reduce(lambda a,b: a+' '+b, map(itemgetter(0), y)))              # 2. Concantenate the strs with same contiguous key.
          for x,y in groupby(myvals, key=lambda e: e[1])]),              # 1. Group by key for which you have to concat str.
        lambda e: e[0])]

# Output:
# [('arrival', ['cool', 'hello hi']), ('no', ['test']), ('yes', ['voo'])]