python operator.itemgetter

 

python四个魔法方法__len__,__getitem__,__setitem__,__delitem_

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1. class DictDemo:  
2.       def __init__(self,key,value):  
3.             self.dict = {}  
4.             self.dict[key] = value  
5.       def __getitem__(self,key):  
6.             return self.dict[key]  
7.       def __setitem__(self,key,value):  
8.             self.dict[key] = value  
9. dictDemo = DictDemo('key0','value0')  
10. print(dictDemo['key0']) #value0  
11. dictDemo['key1'] = 'value1'  
12. print(dictDemo['key1']) #value1  

import weakref

实现weakref.ValueDict()    descriptor  重写 set方法                               

import weakref  #实现通过id获取object_id2obj_dict = weakref.WeakValueDictionary()def remember(obj):    oid = id(obj)    _id2obj_dict[oid] = obj    return oiddef id2obj(oid):    return _id2obj_dict[oid]

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operator is a built-in module providing a set of convenient operators. In two wordsoperator.itemgetter(n) constructs a callable that assumes iterable object (list, tuple, set) as input an fetches n-th element out of it.

So, you can't use key=a[x][1] there, because python have no idea what x is. Instead, you could use alambda function (elem is just a variable name, no magic here):

a.sort(key=lambda elem: elem[1])

Or just ordinary function:

def get_second_elem(iterable):    return iterable[1]a.sort(key=get_second_elem)

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• >>> students = ['dave', 'john', 'jane']>>> newgrades = {'john': 'F', 'jane':'A', 'dave': 'C'}>>> sorted(students, key=newgrades.__getitem__)['jane', 'dave', 'john']

>>> sorted(student_tuples, key=itemgetter(2), reverse=True)[('john', 'A', 15), ('jane', 'B', 12), ('dave', 'B', 10)]>>> sorted(student_objects, key=attrgetter('age'), reverse=True)[('john', 'A', 15), ('jane', 'B', 12), ('dave', 'B', 10)]