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

> squares = [x**2 for x in range(1,10)]
> squares
[1, 4, 9, 16, 25, 36, 49, 64, 81]

aliquot = []

for n in range(1,100):
  if n%3 == 0:
    aliquot.append(n)

print aliquot

> aliquot = [n for n in range(1,100) if n%3==0]
> aliquot
[3, 6, 9, 12, 15, 18, 21, 24, 27, 30, 33, 36, 39, 42, 45, 48, 51, 54, 57, 60, 63, 66, 69, 72, 75, 78, 81, 84, 87, 90, 93, 96, 99]

> mybag = [' glass',' apple','green leaf ']  #有的前面有空格，有的后面有空格
> [one.strip() for one in mybag]       #去掉元素前后的空格
['glass', 'apple', 'green leaf']

> for i in range(len(week)):
...   print week[i]+' is '+str(i)   #注意，i是int类型，如果和前面的用+连接，必须是str类型
... 
monday is 0
sunday is 1
friday is 2

> for (i,day) in enumerate(week):
...   print day+' is '+str(i)
... 
monday is 0
sunday is 1
friday is 2

> seasons = ['Spring', 'Summer', 'Fall', 'Winter']
> list(enumerate(seasons))
[(0, 'Spring'), (1, 'Summer'), (2, 'Fall'), (3, 'Winter')]
> list(enumerate(seasons, start=1))
[(1, 'Spring'), (2, 'Summer'), (3, 'Fall'), (4, 'Winter')]