if len(mylist):
  # Do something with my list
else:
  # The list is empty

if mylist:
  # Do something with my list
else:
  # The list is empty

i = 0
for element in mylist:
  # Do something with i and element
  i += 1

for i, element in enumerate(mylist):
  # Do something with i and element
  pass

class Person(object):
  def __init__(self, age):
    self.age = age
 
persons = [Person(age) for age in (14, 78, 42)]

def get_sort_key(element):
  return element.age
 
for element in sorted(persons, key=get_sort_key):
  print "Age:", element.age

from operator import attrgetter
 
for element in sorted(persons, key=attrgetter('age')):
  print "Age:", element.age

class Person(object):
  def __init__(self, age):
    self.age = age
 
persons = [Person(age) for age in (78, 14, 78, 42, 14)]

persons_by_age = {}
 
for person in persons:
  age = person.age
  if age in persons_by_age:
    persons_by_age[age].append(person)
  else:
    persons_by_age[age] = [person]
 
assert len(persons_by_age[78]) == 2

from collections import defaultdict
 
persons_by_age = defaultdict(list)
 
for person in persons:
  persons_by_age[person.age].append(person)