# Python小技巧和有趣的内置函数

## Python小技巧和有趣的内置函数

### 1.1 拆箱

``````>>> a, b, c = 1, 2, 3
>>> a, b, c
(1, 2, 3)
>>> a, b, c = [1, 2, 3]
>>> a, b, c
(1, 2, 3)
>>> a, b, c = (2 * i + 1  for i in range(3))
>>> a, b, c
(1, 3, 5)
>>> a, (b, c), d = [1, (2, 3), 4]
>>> a
1
>>> b
2
>>> c
3
>>> d
4
``````

### 1.2 拆箱变量交换

``````>>> a, b = 1, 2
>>> a, b = b, a
>>> a, b
(2, 1)
``````

### 1.3 扩展拆箱(只兼容python3)

``````>>> a, *b, c = [1, 2, 3, 4, 5]
>>> a
1
>>> b
[2, 3, 4]
>>> c
5
``````

### 1.4 负数索引

``````>>> a = [0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10]
>>> a[-1]
10
>>> a[-3]
8
``````

### 1.5 切割列表

``````>>> a = [0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10]
>>> a = [0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10]
>>> a[2:8]
[2, 3, 4, 5, 6, 7]
``````

### 1.6 负数索引切割列表

``````>>> a = [0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10]
>>> a[-4:-2]
[7, 8]
``````

### 1.7指定步长切割列表

``````>>> a = [0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10]
>>> a[::2]
[0, 2, 4, 6, 8, 10]
>>> a[::3]
[0, 3, 6, 9]
>>> a[2:8:2]
[2, 4, 6]
``````

### 1.8 负数步长切割列表

``````>>> a = [0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10]
>>> a[::-1]
[10, 9, 8, 7, 6, 5, 4, 3, 2, 1, 0]
>>> a[::-2]
[10, 8, 6, 4, 2, 0]
``````

### 1.9 列表切割赋值

``````>>> a = [1, 2, 3, 4, 5]
>>> a[2:3] = [0, 0]
>>> a
[1, 2, 0, 0, 4, 5]
>>> a[1:1] = [8, 9]
>>> a
[1, 8, 9, 2, 0, 0, 4, 5]
>>> a[1:-1] = []
>>> a
[1, 5]
``````

### 1.10 命名列表切割方式

``````>>> a = [0, 1, 2, 3, 4, 5]
>>> LASTTHREE = slice(-3, None)
>>> LASTTHREE
slice(-3, None, None)
>>> a[LASTTHREE]
[3, 4, 5]
``````

### 1.11 列表以及迭代器的压缩和解压缩

``````>>> a = [1, 2, 3]
>>> b = ['a', 'b', 'c']
>>> z = zip(a, b)
>>> z
[(1, 'a'), (2, 'b'), (3, 'c')]
>>> zip(*z)
[(1, 2, 3), ('a', 'b', 'c')]
``````

### 1.12 列表相邻元素压缩器

``````>>> a = [1, 2, 3, 4, 5, 6]
>>> zip(*([iter(a)] * 2))
[(1, 2), (3, 4), (5, 6)]
>>> group_adjacent = lambda a, k: zip(*([iter(a)] * k))
[(1, 2, 3), (4, 5, 6)]
[(1, 2), (3, 4), (5, 6)]
[(1,), (2,), (3,), (4,), (5,), (6,)]

>>> zip(a[::2], a[1::2])
[(1, 2), (3, 4), (5, 6)]

>>> zip(a[::3], a[1::3], a[2::3])
[(1, 2, 3), (4, 5, 6)]

>>> group_adjacent = lambda a, k: zip(*(a[i::k] for i in range(k)))
[(1, 2, 3), (4, 5, 6)]
[(1, 2), (3, 4), (5, 6)]
[(1,), (2,), (3,), (4,), (5,), (6,)]
``````

### 1.13 在列表中用压缩器和迭代器滑动取值窗口

``````>>> def n_grams(a, n):
... z = [iter(a[i:]) for i in range(n)]
... return zip(*z)
...
>>> a = [1, 2, 3, 4, 5, 6]
>>> n_grams(a, 3)
[(1, 2, 3), (2, 3, 4), (3, 4, 5), (4, 5, 6)]
>>> n_grams(a, 2)
[(1, 2), (2, 3), (3, 4), (4, 5), (5, 6)]
>>> n_grams(a, 4)
[(1, 2, 3, 4), (2, 3, 4, 5), (3, 4, 5, 6)]
``````

### 用压缩器反转字典

``````>>> m = {'a': 1, 'b': 2, 'c': 3, 'd': 4}
>>> m.items()
[('a', 1), ('c', 3), ('b', 2), ('d', 4)]
>>> zip(m.values(), m.keys())
[(1, 'a'), (3, 'c'), (2, 'b'), (4, 'd')]
>>> mi = dict(zip(m.values(), m.keys()))
>>> mi
{1: 'a', 2: 'b', 3: 'c', 4: 'd'}
``````

### 1.15 列表展开

``````>>> a = [[1, 2], [3, 4], [5, 6]]
>>> list(itertools.chain.from_iterable(a))
[1, 2, 3, 4, 5, 6]

>>> sum(a, [])
[1, 2, 3, 4, 5, 6]

>>> [x for l in a for x in l]
[1, 2, 3, 4, 5, 6]

>>> a = [[[1, 2], [3, 4]], [[5, 6], [7, 8]]]
>>> [x for l1 in a for l2 in l1 for x in l2]
[1, 2, 3, 4, 5, 6, 7, 8]

>>> a = [1, 2, [3, 4], [[5, 6], [7, 8]]]
>>> flatten = lambda x: [y for l in x for y in flatten(l)] if type(x) is list else [x]
>>> flatten(a)
[1, 2, 3, 4, 5, 6, 7, 8]
``````

### 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 字典推导

``````>>> 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 用字典推导反转字典

``````>>> m = {'a': 1, 'b': 2, 'c': 3, 'd': 4}
>>> m
{'d': 4, 'a': 1, 'b': 2, 'c': 3}
>>> {v: k for k, v in m.items()}
{1: 'a', 2: 'b', 3: 'c', 4: 'd'}
``````

### 1.19 命名元组

``````>>> 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
``````

### all(iterable)

``````_all = True
for item in iterable:
if  not item:
_all = False
break
if _all:
# do stuff
``````

``````if all(iterable):
# do stuff
``````

### any(iterable)

``````_any = False
for item in iterable:
if item:
_any = True
break
if _any:
# do stuff
``````

``````if any(iterable):
# do stuff
``````

### cmp(x, y)

``````def compare(x,y):
if x < y:
return -1
elif x == y:
return  0
else:
return  1
``````

### dict([arg])

arg 通常是未知的，但是它很方便！比如说，如果我们想把一个含两个元组的列表转换成一个字典，我们可以这么做。

``````l = [('Knights', 'Ni'), ('Monty', 'Python'), ('SPAM', 'SPAAAM')]
d = dict()
for tuple in l:
d[tuple[0]] = tuple[1]
# {'Knights': 'Ni', 'Monty': 'Python', 'SPAM': 'SPAAAM'}
``````

``````l = [('Knights', 'Ni'), ('Monty', 'Python'), ('SPAM', 'SPAAAM')]
d = dict(l) # {'Knights': 'Ni', 'Monty': 'Python', 'SPAM': 'SPAAAM'}
``````

### enumerate(iterable [,start=0])

``````for i in range(len(list)):
# do stuff with list[i], for example, print it
print i, list[i]
``````

``````for i, item in enumerate(list):
# so stuff with item, for example print it
print i, item
``````

### isinstance(object, classinfo)

``````if type(obj) == type(dict):
# do stuff
elif type(obj) == type(list):
# do other stuff
...
``````

``````if isinstance(obj, dict):
# do stuff
elif isinstance(obj, list):
# do other stuff
...
``````

### pow(x, y [,z])

``````mod = (x ** y) % z
``````

### zip([iterable, ])

``````l1 = ('You gotta', 'the')
l2 = ('love', 'built-in')
out = []
if len(l1) == len(l2):
for i in range(len(l1)):
out.append((l1[i], l2[i]))
# out = [('You gotta', 'love'), ('the', 'built-in)]
``````

``````l1 = ['You gotta', 'the']
l2 = ['love', 'built-in']
out = zip(l1, l2) # [('You gotta', 'love'), ('the', 'built-in)]
``````

``````print zip(*out)
# [('You gotta', 'the'), ('love', 'built-in')]
``````

### 结论

Python 内置函数很方便，它们很快并且经过了优化，所以它们可能效率更高。

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