pythonislice_python基础知识

字符串格式化

>>> "{key}&＃61;{value}".format(key&＃61;"a", value&＃61;10) # 使⽤命名参数

&＃39;a&＃61;10&＃39;

>>> "[{0:<10}], [{0:^10}], [{0:*>10}]".format("a") # 左中右对⻬

&＃39;[a ], [ a ], [*********a]&＃39;

>>> "{0.platform}".format(sys) # 成员

&＃39;darwin&＃39;

>>> "{0[a]}".format(dict(a&＃61;10, b&＃61;20)) # 字典

&＃39;10&＃39;

>>> "{0[5]}".format(range(10)) # 列表

&＃39;5&＃39;

>>> "My name is {0} :-{{}}".format(&＃39;Fred&＃39;) # 真得想显示{},需要双{}

&＃39;My name is Fred :-{}&＃39;

>>> "{0!r:20}".format("Hello")

"&＃39;Hello&＃39; "

>>> "{0!s:20}".format("Hello")

&＃39;Hello &＃39;

>>> "Today is: {0:%a %b %d %H:%M:%S %Y}".format(datetime.now())

&＃39;Today is: Mon Mar 31 23:59:34 2014&＃39;

列表去重

>>> l &＃61; [1, 2, 2, 3, 3, 3]

>>> {}.fromkeys(l).keys()

[1, 2, 3] # 列表去重(1)

>>> list(set(l)) # 列表去重(2)

[1, 2, 3]

In [2]: %timeit list(set(l))

1000000 loops, best of 3: 956 ns per loop

In [3]: %timeit {}.fromkeys(l).keys()

1000000 loops, best of 3: 1.1 µs per loop

In [4]: l &＃61; [random.randint(1, 50) for i in range(10000)]

In [5]: %timeit list(set(l))

1000 loops, best of 3: 271 µs per loop

In [6]: %timeit {}.fromkeys(l).keys()

1000 loops, best of 3: 310 µs per loop

PS: 在字典较大的情况下, 列表去重(1)略慢了

super 当子类调用父类属性时一般的做法是这样

>>> class LoggingDict(dict):

... def __setitem__(self, key, value):

... print(&＃39;Setting {0} to {1}&＃39;.format(key, value))

... dict.__setitem__(self, key, value)

问题是假如你继承的不是dict而是其他,那么就要修改2处,其实可以这样

>>> class LoggingDict(dict):

... def __setitem__(self, key, value):

... print(&＃39;Setting {0} to {1}&＃39;.format(key, value))

... super(LoggingDict, self).__setitem__(key, value)

PS: 感觉super自动找到了LoggingDict的父类(dict)&＃xff0c;然后把self转化为其实例

斐波那契数列

>>> import itertools

>>>

>>> def fib():

... a, b &＃61; 0, 1

... while 1:

... yield b

... a, b &＃61; b, a &＃43; b

...

>>>

>>> print list(itertools.islice(fib(), 10))

[1, 1, 2, 3, 5, 8, 13, 21, 34, 55]

看到这里, 就得说说contextmanager

&＃64;contextlib.contextmanager

def some_generator():

try:

yield

finally:

with some_generator() as :

也就是:

try:

&＃61;

finally:

contextmanager例子(一)

>>> lock &＃61; threading.Lock()

>>> &＃64;contextmanager

... def openlock():

... print(&＃39;Acquire&＃39;)

... lock.acquire()

... yield

... print(&＃39;Releasing&＃39;)

... lock.release()

...

>>> with openlock():

... print(&＃39;Lock is locked: {}&＃39;.format(lock.locked()))

... print &＃39;Do some stuff&＃39;

...

Acquire

Lock is locked: True

Do some stuff

Releasing

__slots__ 大量属性时减少内存占用

>>> class User(object):

... __slots__ &＃61; ("name", "age")

... def __init__(self, name, age):

... self.name &＃61; name

... self.age &＃61; age

...

>>> u &＃61; User("Dong", 28)

>>> hasattr(u, "__dict__")

False

>>> u.title &＃61; "xxx"

Traceback (most recent call last):

File "", line 1, in

AttributeError: &＃39;User&＃39; object has no attribute &＃39;title&＃39;

模块: itertools(一)

>>> def chunker(items, chunk_size):

... for _key, group in itertools.groupby(

... enumerate(items), lambda x: x[0] // chunk_size):

... yield [g[1] for g in group]

>>> for i in chunker(range(10), 4):

... print list(i)

[0, 1, 2, 3]

[4, 5, 6, 7]

[8, 9]

>>> l &＃61; [(1, 10), (2, 10), (3, 20), (4, 20)]

>>> for key, group in itertools.groupby(l, lambda t: t[1]):

... print(key, list(group))

(10, [(1, 10), (2, 10)])

(20, [(3, 20), (4, 20)])

chain: 把多个迭代器合并成一个迭代器

islice: 迭代器分片islice(func, start, end, step)