python lib 之 operator

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)