Chain a list of tuples.

sorting_fun_01.py

# We can sort using a custom adjacency index that allows us to pick the starting node.
tickets = [('PHL','NYC'),('IAD','PHL'),('SFO','IAD'),('NYC','SFO')]
sortedTickets = []
index = {}

for t in tickets:
    if t[0] not in index:
        index[t[0]] = t

def getNext(ticket):
    return index[ticket[1]]

currentTicket = tickets[0]

while len(tickets):
    sortedTickets.append(currentTicket)
    tickets.remove(currentTicket)
    currentTicket= getNext(currentTicket)

print sortedTickets
#[('PHL', 'NYC'), ('NYC', 'SFO'), ('SFO', 'IAD'), ('IAD', 'PHL')]

sorting_fun_02.py

# We can sort using a custom adjacency index that allows us to pick the starting node.
tickets = [('PHL','NYC'),('IAD','PHL'),('SFO','IAD'),('NYC','SFO')]
sortedTickets = []
index = {}

for t in tickets:
    if t[0] not in index:
        index[t[0]] = {}
    index[t[0]]['src'] = t

    if t[1] not in index:
        index[t[1]] = {}
    index[t[1]]['dest'] = t

def getNext(ticket):
    return index[ticket[1]]['src']


def getPrev(ticket):
    return index[ticket[0]]['dest']

currentTicket = tickets[0]

while len(tickets):
    sortedTickets.append(currentTicket)
    tickets.remove(currentTicket)
    currentTicket= getNext(currentTicket)


print sortedTickets
#[('PHL', 'NYC'), ('NYC', 'SFO'), ('SFO', 'IAD'), ('IAD', 'PHL')]

sorting_fun_03.py

# We can sort using a comparison function but we have no control over starting node.
tickets = [('PHL','NYC'),('IAD','PHL'),('SFO','IAD'),('NYC','SFO')]

def compareTix(a,b):
    if a[0] == b[1]:
        return 1
    if a[1] == b[0]:
        return -1
    return 0

tickets.sort(cmp=compareTix)

print tickets
#[('NYC', 'SFO'), ('SFO', 'IAD'), ('IAD', 'PHL'), ('PHL', 'NYC')]

sorting_fun_04.py

import networkx as nx

# Using a graph we can find the cycle for a given start node.
G=nx.Graph([('PHL','NYC'),('IAD','PHL'),('SFO','IAD'),('NYC','SFO')])
print list(nx.cycle_basis(G,'PHL'))
# [['NYC', 'SFO', 'IAD', 'PHL']]

# Using a directed graph we can find the cycles, but can't pick the start node.
DG=nx.DiGraph([('PHL','NYC'),('IAD','PHL'),('SFO','IAD'),('NYC','SFO')])
print list(nx.simple_cycles(DG))
# [['NYC', 'SFO', 'IAD', 'PHL']]