python lib 之 operator

常青树

　　operator module在使用内置函数如map,itertools.groupby,sorted排序使用DSU技巧时，经常用到；这里面最经常用到的两个函数是：


operator.
attrgetter
(

attr
[
, args...
]
)


这个函数返回一个可调用对象（Callable Objects）,取此对象的attr属性值；如果参数是多个属性，则返回的是属性值tuple.
例如：f
=
attrgetter('name')

,则 f(b)

返回的是 b.name

. f
=
attrgetter('name',
'date')

,则 f(b)

返回的是

(b.name,
b.date)

. Equivalent to:

def attrgetter(*items):
if len(items) == 1:
attr = items[0]
def g(obj):
return resolve_attr(obj, attr)
else:
def g(obj):
return tuple(resolve_att(obj, attr) for attr in items)
return g
def resolve_attr(obj, attr):
for name in attr.split("."):
obj = getattr(obj, name)
return obj

resolve_attr是用来处理属性名有.情 况；f
=
attrgetter('date.month')

,调用 f(b)

返回的是 b.date.month

.
应用举例DSU：
import operator
def sort_by_attr(seq, attr):
return sorted(seq, key=operator.attrgetter(attr))
def sort_by_attr_inplace(lst, attr):
lst.sort(key=operator.attrgetter(attr))



operator.
itemgetter
(

item
[
, args...
]
)


　　Return a callable object that fetches item
from its operand using the operand’s __getitem__()

method. If multiple items are specified, returns a tuple of lookup values. Equivalent to:

def itemgetter(*items):
if len(items) == 1:
item = items[0]
def g(obj):
return obj[item]
else:
def g(obj):
return tuple(obj[item] for item in items)
return g



　　The items can be any type accepted by the operand’s __getitem__()

method. Dictionaries accept any hashable value. Lists, tuples, and strings accept an index or a slice:

>>>
itemgetter
(
1
)(
'ABCDEFG'
)
'B'
>>>
itemgetter
(
1
,
3
,
5
)(
'ABCDEFG'
)
('B', 'D', 'F')
>>>
itemgetter
(
slice
(
2
,
None
))(
'ABCDEFG'
)
'CDEFG'






from operator import itemgetter
def dict_items_sorted_by_value(d, reverse=False):
return sorted(d.iteritems( ), key=itemgetter(1), reverse=reverse)
class Hist(object):
def __init__(self,dict={}):
self.adict = {}
def add(self,item,increment = 1):
self.adict[item] = increment + self.adict.get(item,0)
def counts(self,reverse=False):
return dict_items_sorted_by_value(self.adict,reverse)
if __name__ == '__main__':
sentence = ''' Hello there this is a test.  Hello there this was a test,
but now it is not. '''
words = sentence.split( )
c = Hist( )
for word in words: c.add(word)
print "Ascending count:"
print c.counts( )
print "Descending count:"
print c.counts(reverse=True)