# 常用内建模块
### 知识点总结
> * datetime
1. datetime.now() # 获取当前datetime
datetime.now()返回当前日期和时间,其类型是datetime。
2. datetime(2015, 4, 19, 12, 20) # 用指定日期时间创建datetime
>>> dt = datetime(2015, 4, 19, 12, 20) # 用指定日期时间创建datetime
>>> print(dt)
2015-04-19 12:20:00
3. datetime转换为timestamp
>>> dt = datetime(2015, 4, 19, 12, 20) # 用指定日期时间创建datetime
>>> dt.timestamp() # 把datetime转换为timestamp
1429417200.0
timestamp是一个浮点数。
4. fromtimestamp()
>>> t = 1429417200.0
>>> print(datetime.fromtimestamp(t)) # 本地时间
2015-04-19 12:20:00
>>> print(datetime.utcfromtimestamp(t)) # UTC时间
2015-04-19 04:20:00
timestamp是一个浮点数,它没有时区的概念,而datetime是有时区的
5. str转换为datetime
>>> cday = datetime.strptime('2015-6-1 18:19:59', '%Y-%m-%d %H:%M:%S')
>>> print(cday)
2015-06-01 18:19:59
6. datetime转换为str
>>> now = datetime.now()
>>> print(now.strftime('%a, %b %d %H:%M'))
Mon, May 05 16:28
7. datetime加减
now = datetime.now()
now + timedelta(hours=10)
now - timedelta(days=1)
now + timedelta(days=2, hours=12)
8. 时区转换
astimezone()
> * collections
1. namedtuple
namedtuple是一个函数,它用来创建一个自定义的tuple对象,并且规定了tuple元素的个数,
并可以用属性而不是索引来引用tuple的某个元素
Point = namedtuple('Point', ['x', 'y'])
Circle = namedtuple('Circle', ['x', 'y', 'r'])
2. deque
deque是为了高效实现插入和删除操作的双向列表,适合用于队列和栈:
deque除了实现list的append()和pop()外,还支持appendleft()和popleft()
3. defaultdict
使用dict时,如果引用的Key不存在,返回一个默认值,就可以用defaultdict:
>>> dd = defaultdict(lambda: 'N/A')
>>> dd['key1'] = 'abc'
>>> dd['key1'] # key1存在
'abc'
>>> dd['key2'] # key2不存在,返回默认值
'N/A'
4. OrderedDict
使用dict时,Key是无序的。在对dict做迭代时,我们无法确定Key的顺序。
如果要保持Key的顺序,可以用OrderedDict:
>>> d = dict([('a', 1), ('b', 2), ('c', 3)])
>>> d # dict的Key是无序的
{'a': 1, 'c': 3, 'b': 2}
>>> od = OrderedDict([('a', 1), ('b', 2), ('c', 3)])
>>> od # OrderedDict的Key是有序的
OrderedDict([('a', 1), ('b', 2), ('c', 3)])
注意,OrderedDict的Key会按照插入的顺序排列,不是Key本身排序:
5. ChainMap
ChainMap可以把一组dict串起来并组成一个逻辑上的dict。
ChainMap本身也是一个dict,但是查找的时候,会按照顺序在内部的dict依次查找。
6. Counter
Counter是一个简单的计数器,例如,统计字符出现的个数:
>>> c = Counter()
>>> for ch in 'programming':
... c[ch] = c[ch] + 1
...
>>> c
Counter({'g': 2, 'm': 2, 'r': 2, 'a': 1, 'i': 1, 'o': 1, 'n': 1, 'p': 1})
> * base64
1. Base64是一种用64个字符来表示任意二进制数据的方法。
>>> base64.b64encode(b'binary\x00string')
b'YmluYXJ5AHN0cmluZw=='
>>> base64.b64decode(b'YmluYXJ5AHN0cmluZw==')
b'binary\x00string'
2. 由于标准的Base64编码后可能出现字符+和/,在URL中就不能直接作为参数,
所以又有一种"url safe"的base64编码,其实就是把字符+和/分别变成-和_:
>>> base64.b64encode(b'i\xb7\x1d\xfb\xef\xff')
b'abcd++//'
>>> base64.urlsafe_b64encode(b'i\xb7\x1d\xfb\xef\xff')
b'abcd--__'
>>> base64.urlsafe_b64decode('abcd--__')
b'i\xb7\x1d\xfb\xef\xff'
3. Base64是一种通过查表的编码方法,不能用于加密,即使使用自定义的编码表也不行。
Base64适用于小段内容的编码,比如数字证书签名、Cookie的内容等。
> * struct
1. struct
可以将bytes和其他二进制数据类型的转换
2. pack
struct的pack函数把任意数据类型变成bytes:
>>> import struct
>>> struct.pack('>I', 10240099)
b'\x00\x9c@c'
pack的第一个参数是处理指令,'>I'的意思是:
>表示字节顺序是big-endian,也就是网络序,I表示4字节无符号整数。
后面的参数个数要和处理指令一致。
unpack把bytes变成相应的数据类型:
>>> struct.unpack('>IH', b'\xf0\xf0\xf0\xf0\x80\x80')
(4042322160, 32896)
根据>IH的说明,后面的bytes依次变为I:4字节无符号整数和H:2字节无符号整数。
> * hashlib
1. MD5
md5 = hashlib.md5()
md5.update('how to use md5 in python hashlib?'.encode('utf-8'))
print(md5.hexdigest())
d26a53750bc40b38b65a520292f69306
MD5是最常见的摘要算法,速度很快,生成结果是固定的128 bit字节,通常用一个32位的16进制字符串表示。
2. SHA1
sha1 = hashlib.sha1()
sha1.update('how to use sha1 in '.encode('utf-8'))
sha1.update('python hashlib?'.encode('utf-8'))
print(sha1.hexdigest())
SHA1的结果是160 bit字节,通常用一个40位的16进制字符串表示。
> * hmac
1. md5(message + salt)
2. hmac
>>> message = b'Hello, world!'
>>> key = b'secret'
>>> h = hmac.new(key, message, digestmod='MD5')
>>> # 如果消息很长,可以多次调用h.update(msg)
>>> h.hexdigest()
'fa4ee7d173f2d97ee79022d1a7355bcf'
可见使用hmac和普通hash算法非常类似。
hmac输出的长度和原始哈希算法的长度一致。
需要注意传入的key和message都是bytes类型,str类型需要首先编码为bytes。
> * itertools
1. count()
natuals = itertools.count(1)
2. cycle()
cs = itertools.cycle('ABC') # 注意字符串也是序列的一种
3. repeat()
ns = itertools.repeat('A', 3)
4. takewhile()
根据条件判断来截取出一个有限的序列:
>>> natuals = itertools.count(1)
>>> ns = itertools.takewhile(lambda x: x <= 10, natuals)
>>> list(ns)
[1, 2, 3, 4, 5, 6, 7, 8, 9, 10]
5. chain()
把一组迭代对象串联起来,形成一个更大的迭代器:
>>> for c in itertools.chain('ABC', 'XYZ'):
... print(c)
# 迭代效果:'A' 'B' 'C' 'X' 'Y' 'Z'
6. groupby()
把迭代器中相邻的重复元素挑出来放在一起:
>>> for key, group in itertools.groupby('AAABBBCCAAA'):
... print(key, list(group))
...
A ['A', 'A', 'A']
B ['B', 'B', 'B']
C ['C', 'C']
A ['A', 'A', 'A']
挑选规则是通过函数完成的:
>>> for key, group in itertools.groupby('AaaBBbcCAAa', lambda c: c.upper()):
... print(key, list(group))
...
A ['A', 'a', 'a']
B ['B', 'B', 'b']
C ['c', 'C']
A ['A', 'A', 'a']
> * contextlib
1. 实际上,任何对象,只要正确实现了上下文管理,就可以用于with语句。
实现上下文管理是通过__enter__和__exit__这两个方法实现的
class Query(object):
def __init__(self, name):
self.name = name
def __enter__(self):
print('Begin')
return self
def __exit__(self, exc_type, exc_value, traceback):
if exc_type:
print('Error')
else:
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