从我开始学习Python时我就决定维护一个经常使用的“窍门”列表。不论何时当我看到一段让我觉得“酷，这样也行！”的代码时（在一个例子中、在StackOverflow、在开源码软件中，等等），我会尝试它直到理解它，然后把它添加到列表中。这篇文章是清理过列表的一部分。如果你是一个有经验的Python程序员，尽管你可能已经知道一些，但你仍能发现一些你不知道的。如果你是一个正在学习Python的C、C++或Java程序员，或者刚开始学习编程，那么你会像我一样发现它们中的很多非常有用。
每个窍门或语言特性只能通过实例来验证，无需过多解释。虽然我已尽力使例子清晰，但它们中的一些仍会看起来有些复杂，这取决于你的熟悉程度。所以如果看过例子后还不清楚的话，标题能够提供足够的信息让你通过Google获取详细的内容。
列表按难度排序，常用的语言特征和技巧放在前面。
1.15 摊平列表:
>>> a = [[1,2],[3,4],[5,6]]
>>> list(itertools.chain.from_iterable(a))
[1,2,3,4,5,6]
>>> sum(a,[])
[1,6]
>>> [x for l in a for x in l]
[1,6]
>>> a = [[[1,4]],[[5,6],[7,8]]]
>>> [x for l1 in a for l2 in l1 for x in l2]
[1,6,7,8]
>>> a = [1,8]]]
>>> flatten = lambda x: [y for l in x for y in flatten(l)] if type(x) is list else [x]
>>> flatten(a)
[1,8]
注意: 根据Python的文档，itertools.chain.from_iterable是首选。
1.16 生成器表达式
>>> g = (x ** 2 for x in xrange(10))
>>> next(g)
0
>>> next(g)
1
>>> next(g)
4
>>> next(g)
9
>>> sum(x ** 3 for x in xrange(10))
2025
>>> sum(x ** 3 for x in xrange(10) if x % 3 == 1)
408
1.17 迭代字典
>>> m = {x: x ** 2 for x in range(5)}
>>> m
{0: 0,1: 1,2: 4,3: 9,4: 16}
>>> m = {x: 'A' + str(x) for x in range(10)}
>>> m
{0: 'A0',1: 'A1',2: 'A2',3: 'A3',4: 'A4',5: 'A5',6: 'A6',7: 'A7',8: 'A8',9: 'A9'}
1.18 通过迭代字典反转字典
>>> m = {'a': 1,'b': 2,'c': 3,'d': 4}
>>> m
{'d': 4,'a': 1,'c': 3}
>>> {v: k for k,v in m.items()}
{1: 'a',2: 'b',3: 'c',4: 'd'}
1.19 命名序列 (collections.namedtuple)
>>> Point = collections.namedtuple('Point',['x','y'])
>>> p = Point(x=1.0,y=2.0)
>>> p
Point(x=1.0,y=2.0)
>>> p.x
1.0
>>> p.y
2.0
1.20 命名列表的继承:
>>> class Point(collections.namedtuple('PointBase','y'])):
... __slots__ = ()
... def __add__(self,other):
... return Point(x=self.x + other.x,y=self.y + other.y)
...
>>> p = Point(x=1.0,y=2.0)
>>> q = Point(x=2.0,y=3.0)
>>> p + q
Point(x=3.0,y=5.0)
1.21 集合及集合操作
>>> A = {1,3}
>>> A
set([1,3])
>>> B = {3,7}
>>> B
set([3,7])
>>> A | B
set([1,7])
>>> A & B
set([3])
>>> A - B
set([1,2])
>>> B - A
set([4,7])
>>> A ^ B
set([1,7])
>>> (A ^ B) == ((A - B) | (B - A))
True
1.22 多重集及其操作 (collections.Counter)
>>> A = collections.Counter([1,2])
>>> B = collections.Counter([2,3])
>>> A
Counter({2: 2,1: 1})
>>> B
Counter({2: 2,3: 1})
>>> A | B
Counter({2: 2,3: 1})
>>> A & B
Counter({2: 2})
>>> A + B
Counter({2: 4,3: 1})
>>> A - B
Counter({1: 1})
>>> B - A
Counter({3: 1})
1.23 迭代中最常见的元素 (collections.Counter)
>>> A = collections.Counter([1,1,7])
>>> A
Counter({3: 4,1: 2,2: 2,4: 1,5: 1,6: 1,7: 1})
>>> A.most_common(1)
[(3,4)]
>>> A.most_common(3)
[(3,4),(1,2),(2,2)]
1.24 双端队列 (collections.deque)
>>> Q = collections.deque()
>>> Q.append(1)
>>> Q.appendleft(2)
>>> Q.extend([3,4])
>>> Q.extendleft([5,6])
>>> Q
deque([6,4])
>>> Q.pop()
4
>>> Q.popleft()
6
>>> Q
deque([5,3])
>>> Q.rotate(3)
>>> Q
deque([2,5])
>>> Q.rotate(-3)
>>> Q
deque([5,3])
1.25 有最大长度的双端队列 (collections.deque)
>>> last_three = collections.deque(maxlen=3)
>>> for i in xrange(10):
... last_three.append(i)
... print ','.join(str(x) for x in last_three)
...
0
0,1
0,2
1,3
2,4
3,5
4,6
5,7
6,8
7,8,9
1.26 字典排序 (collections.OrderedDict)
>>> m = dict((str(x),x) for x in range(10))
>>> print ','.join(m.keys())
1,9,8
>>> m = collections.OrderedDict((str(x),'.join(m.keys())
0,9
>>> m = collections.OrderedDict((str(x),x) for x in range(10,-1))
>>> print ','.join(m.keys())
10,1
1.27 缺省字典 (collections.defaultdict)
>>> m = dict()
>>> m['a']
Traceback (most recent call last):
File "<stdin>",line 1,in <module>
KeyError: 'a'
>>>
>>> m = collections.defaultdict(int)
>>> m['a']
0
>>> m['b']
0
>>> m = collections.defaultdict(str)
>>> m['a']
''
>>> m['b'] += 'a'
>>> m['b']
'a'
>>> m = collections.defaultdict(lambda: '[default value]')
>>> m['a']
'[default value]'
>>> m['b']
'[default value]'
1.28 用缺省字典表示简单的树
>>> import json
>>> tree = lambda: collections.defaultdict(tree)
>>> root = tree()
>>> root['menu']['id'] = 'file'
>>> root['menu']['value'] = 'File'
>>> root['menu']['menuitems']['new']['value'] = 'New'
>>> root['menu']['menuitems']['new']['onclick'] = 'new();'
>>> root['menu']['menuitems']['open']['value'] = 'Open'
>>> root['menu']['menuitems']['open']['onclick'] = 'open();'
>>> root['menu']['menuitems']['close']['value'] = 'Close'
>>> root['menu']['menuitems']['close']['onclick'] = 'close();'
>>> print json.dumps(root,sort_keys=True,indent=4,separators=(',',': '))
{
"menu": {
"id": "file",
"menuitems": {
"close": {
"onclick": "close();",
"value": "Close"
},
"new": {
"onclick": "new();",
"value": "New"
},
"open": {
"onclick": "open();",
"value": "Open"
}
},
"value": "File"
}
}
(到https://gist.github.com/hrldcpr/2012250查看详情)
1.29 映射对象到唯一的序列数 (collections.defaultdict)
>>> import itertools,collections
>>> value_to_numeric_map = collections.defaultdict(itertools.count().next)
>>> value_to_numeric_map['a']
0
>>> value_to_numeric_map['b']
1
>>> value_to_numeric_map['c']
2
>>> value_to_numeric_map['a']
0
>>> value_to_numeric_map['b']
1
原文链接：https://www.f2er.com/python/527216.html