::-- ZoomQuiet [2007-11-10 07:34:49]
Contents
1. fast UserDict
{{{Jiahua Huang <[email protected]> reply-to [email protected], to "python. cn" <[email protected]>, date Nov 10, 2007 3:28 PM subject [CPyUG:34791] 一行代码让 UserDict.UserDict 的类加速 4 倍 }}} 发现 Python 标准库里好些字典类从 UserDict.UserDict 派生， 而不是从 dict 派生， 是因为 旧版 python 内建类型不能派生子类，
那么这会不会影响速度呢，
先给两个分别继承 UserDict.UserDict 和 dict 的类 URdict, Rdict
>>> import UserDict
>>> class URdict(UserDict.UserDict):
... '''dict can search key by value
... '''
... def indexkey4value(self, value):
... '''search key by value
... >>> rd = Rdict(a='One', b='Other', c='What', d='Why', e='Other')
... >>> rd.indexkey4value('Other')
... 'b'
... '''
... try:
... ind = self.values().index(value)
... return self.keys()[ind]
... except:
... return None
... def key4value(self, svalue):
... '''search key by value
... >>> rd = Rdict(a='One', b='Other', c='What', d='Why', e='Other')
... >>> rd.key4value('Other')
... 'b'
... '''
... for key, value in self.iteritems():
... if value == svalue:
... return key
... def keys4value(self, svalue):
... '''search keys by value
... >>> rd = Rdict(a='One', b='Other', c='What', d='Why', e='Other')
... >>> rd.keys4value('Other')
... ['b', 'e']
... '''
... keys=[]
... for key, value in self.iteritems():
... if value == svalue:
... keys.append(key)
... return keys
...
>>>
>>> class Rdict(dict):
... '''dict can search key by value
... '''
... def indexkey4value(self, value):
... '''search key by value
... >>> rd = Rdict(a='One', b='Other', c='What', d='Why', e='Other')
... >>> rd.indexkey4value('Other')
... 'b'
... '''
... try:
... ind = self.values().index(value)
... return self.keys()[ind]
... except:
... return None
... def key4value(self, svalue):
... '''search key by value
... >>> rd = Rdict(a='One', b='Other', c='What', d='Why', e='Other')
... >>> rd.key4value('Other')
... 'b'
... '''
... for key, value in self.iteritems():
... if value == svalue:
... return key
... def keys4value(self, svalue):
... '''search keys by value
... >>> rd = Rdict(a='One', b='Other', c='What', d='Why', e='Other')
... >>> rd.keys4value('Other')
... ['b', 'e']
... '''
... keys=[]
... for key, value in self.iteritems():
... if value == svalue:
... keys.append(key)
... return keys
...
>>>
>>> import time
>>> def _timeit(_src):
... exec('''
... _t0 = time.time()
... %s
... _t1 = time.time()
... _t3 = _t1 - _t0
... '''%_src)
... return _t3
...
>>> ran = range(100000)
再弄俩实例
>>> u = URdict()
>>> r = Rdict()
看看插入速度
>>> _timeit("for i in ran: u[i]=i")
0.1777961254119873
>>> _timeit("for i in ran: r[i]=i")
0.048948049545288086
看看原始 dict 的速度
>>> _timeit("for i in ran: d[i]=i")
0.041368961334228516可以看到， UserDict.UserDict 确实严重影响速度，
python 标准库里边好多 UserDict 的都应该换成 dict ， 以提高性能
不过，一个个修改 Python 标准库似乎又不合适，
再次使用一招鲜，直接干掉 UserDict
在使用/导入那些模块前先来一行
>>> import UserDict; UserDict.UserDict = dict
完了再导入模块来试试
>>> u = URdict()
>>> _timeit("for i in ran: u[i]=i")
0.042366981506347656一行代码让速度提高 4 倍