Python itertools.groupby example with US state capitals

example.py

from itertools import groupby

state_capitals = ['Montgomery', 'Juneau', 'Phoenix', 'Little Rock', 'Sacramento', 'Denver',
                  'Hartford', 'Dover', 'Tallahassee', 'Atlanta', 'Honolulu', 'Boise', 'Springfield',
                  'Indianapolis', 'Des Moines', 'Topeka', 'Frankfort', 'Baton Rouge', 'Augusta',
                  'Annapolis', 'Boston', 'Lansing', 'Saint Paul', 'Jackson', 'Jefferson City',
                  'Helena', 'Lincoln', 'Carson City', 'Concord', 'Trenton', 'Santa Fe', 'Albany',
                  'Raleigh', 'Bismarck', 'Columbus', 'Oklahoma City', 'Salem', 'Harrisburg',
                  'Providence', 'Columbia', 'Pierre', 'Nashville', 'Austin', 'Salt Lake City',
                  'Montpelier', 'Richmond', 'Olympia', 'Charleston', 'Madison', 'Cheyenne']

for letter, words in groupby(sorted(state_capitals), key=lambda c: c[0]):
    print('{}:\n    {}\n'.format(letter, '\n    '.join(words)))

output.txt

A:
    Albany
    Annapolis
    Atlanta
    Augusta
    Austin

B:
    Baton Rouge
    Bismarck
    Boise
    Boston

C:
    Carson City
    Charleston
    Cheyenne
    Columbia
    Columbus
    Concord

D:
    Denver
    Des Moines
    Dover

F:
    Frankfort

H:
    Harrisburg
    Hartford
    Helena
    Honolulu

I:
    Indianapolis

J:
    Jackson
    Jefferson City
    Juneau

L:
    Lansing
    Lincoln
    Little Rock

M:
    Madison
    Montgomery
    Montpelier

N:
    Nashville

O:
    Oklahoma City
    Olympia

P:
    Phoenix
    Pierre
    Providence

R:
    Raleigh
    Richmond

S:
    Sacramento
    Saint Paul
    Salem
    Salt Lake City
    Santa Fe
    Springfield

T:
    Tallahassee
    Topeka
    Trenton