顯示有限的接口到外部
當發(fā)布python第三方package時, 并不希望代碼中所有的函數(shù)或者class可以被外部import, 在__init__.py中添加__all__屬性,
該list中填寫可以import的類或者函數(shù)名, 可以起到限制的import的作用, 防止外部import其他函數(shù)或者類
#!/usr/bin/env python
# -*- coding: utf-8 -*-
from base import APIBase
from client import Client
from decorator import interface, export, stream
from server import Server
from storage import Storage
from util import (LogFormatter, disable_logging_to_stderr,
enable_logging_to_kids, info)
__all__ = ['APIBase', 'Client', 'LogFormatter', 'Server',
'Storage', 'disable_logging_to_stderr', 'enable_logging_to_kids',
'export', 'info', 'interface', 'stream']
with語句需要支持上下文管理協(xié)議的對象, 上下文管理協(xié)議包含__enter__和__exit__兩個方法. with語句建立運行時上下文需要通過這兩個方法執(zhí)行進入和退出操作.
其中上下文表達式是跟在with之后的表達式, 該表示大返回一個上下文管理對象
# 常見with使用場景
with open("test.txt", "r") as my_file:? # 注意, 是__enter__()方法的返回值賦值給了my_file,
for line in my_file:
print line
詳細原理可以查看這篇文章,淺談 Python 的 with 語句
知道具體原理, 我們可以自定義支持上下文管理協(xié)議的類, 類中實現(xiàn)__enter__和__exit__方法
#!/usr/bin/env python
# -*- coding: utf-8 -*-
class MyWith(object):
def __init__(self):
print "__init__ method"
def __enter__(self):
print "__enter__ method"
return self? # 返回對象給as后的變量
def __exit__(self, exc_type, exc_value, exc_traceback):
print "__exit__ method"
if exc_traceback is None:
print "Exited without Exception"
return True
else:
print "Exited with Exception"
return False
def test_with():
with MyWith() as my_with:
print "running my_with"
print "------分割線-----"
with MyWith() as my_with:
print "running before Exception"
raise Exception
print "running after Exception"
if __name__ == '__main__':
test_with()
執(zhí)行結(jié)果如下:
__init__ method
__enter__ method
running my_with
__exit__ method
Exited without Exception
------分割線-----
__init__ method
__enter__ method
running before Exception
__exit__ method
Exited with Exception
Traceback (most recent call last):
File "bin/python", line 34, in
exec(compile(__file__f.read(), __file__, "exec"))
File "test_with.py", line 33, in
test_with()
File "test_with.py", line 28, in test_with
raise Exception
Exception
證明了會先執(zhí)行__enter__方法, 然后調(diào)用with內(nèi)的邏輯, 最后執(zhí)行__exit__做退出處理, 并且, 即使出現(xiàn)異常也能正常退出
相對filter而言, map和reduce使用的會更頻繁一些,filter正如其名字, 按照某種規(guī)則過濾掉一些元素
#!/usr/bin/env python
# -*- coding: utf-8 -*-
lst = [1, 2, 3, 4, 5, 6]
# 所有奇數(shù)都會返回True, 偶數(shù)會返回False被過濾掉
print filter(lambda x: x % 2 != 0, lst)
#輸出結(jié)果
[1, 3, 5]
當條件滿足時, 返回的為等號后面的變量, 否則返回else后語句
lst = [1, 2, 3]
new_lst = lst[0] if lst is not None else None
print new_lst
# 打印結(jié)果
1
使用裝飾器實現(xiàn)簡單的單例模式
# 單例裝飾器
def singleton(cls):
instances = dict()? # 初始為空
def _singleton(*args, **kwargs):
if cls not in instances:? #如果不存在, 則創(chuàng)建并放入字典
instances[cls] = cls(*args, **kwargs)
return instances[cls]
return _singleton
@singleton
class Test(object):
pass
if __name__ == '__main__':
t1 = Test()
t2 = Test()
# 兩者具有相同的地址
print t1, t2
類中兩種常用的裝飾, 首先區(qū)分一下他們
普通成員函數(shù), 其中第一個隱式參數(shù)為對象
classmethod裝飾器, 類方法(給人感覺非常類似于OC中的類方法), 其中第一個隱式參數(shù)為類
staticmethod裝飾器, 沒有任何隱式參數(shù).python中的靜態(tài)方法類似與C++中的靜態(tài)方法
#!/usr/bin/env python
# -*- coding: utf-8 -*-
class A(object):
# 普通成員函數(shù)
def foo(self, x):
print "executing foo(%s, %s)" % (self, x)
@classmethod? # 使用classmethod進行裝飾
def class_foo(cls, x):
print "executing class_foo(%s, %s)" % (cls, x)
@staticmethod? # 使用staticmethod進行裝飾
def static_foo(x):
print "executing static_foo(%s)" % x
def test_three_method():
obj = A()
# 直接調(diào)用噗通的成員方法
obj.foo("para")? # 此處obj對象作為成員函數(shù)的隱式參數(shù), 就是self
obj.class_foo("para")? # 此處類作為隱式參數(shù)被傳入, 就是cls
A.class_foo("para")? #更直接的類方法調(diào)用
obj.static_foo("para")? # 靜態(tài)方法并沒有任何隱式參數(shù), 但是要通過對象或者類進行調(diào)用
A.static_foo("para")
if __name__ == '__main__':
test_three_method()
# 函數(shù)輸出
executing foo(<__main__.A object at 0x100ba4e10>, para)
executing class_foo(, para)
executing class_foo(, para)
executing static_foo(para)
executing static_foo(para)
定義私有類屬性
將property與裝飾器結(jié)合實現(xiàn)屬性私有化(更簡單安全的實現(xiàn)get和set方法)
#python內(nèi)建函數(shù)
property(fget=None, fset=None, fdel=None, doc=None)
fget是獲取屬性的值的函數(shù),fset是設(shè)置屬性值的函數(shù),fdel是刪除屬性的函數(shù),doc是一個字符串(like a comment).從實現(xiàn)來看躬充，這些參數(shù)都是可選的
property有三個方法getter(),setter()和delete()來指定fget, fset和fdel事期。 這表示以下這行
class Student(object):
@property? #相當于property.getter(score) 或者property(score)
def score(self):
return self._score
@score.setter #相當于score = property.setter(score)
def score(self, value):
if not isinstance(value, int):
raise ValueError('score must be an integer!')
if value < 0 or value > 100:
raise ValueError('score must between 0 ~ 100!')
self._score = value
通過yield和__iter__的結(jié)合, 我們可以把一個對象變成可迭代的
通過__str__的重寫, 可以直接通過想要的形式打印對象
#!/usr/bin/env python
# -*- coding: utf-8 -*-
class TestIter(object):
def __init__(self):
self.lst = [1, 2, 3, 4, 5]
def read(self):
for ele in xrange(len(self.lst)):
yield ele
def __iter__(self):
return self.read()
def __str__(self):
return ','.join(map(str, self.lst))
__repr__ = __str__
def test_iter():
obj = TestIter()
for num in obj:
print num
print obj
if __name__ == '__main__':
test_iter()
partial使用上很像C++中仿函數(shù)(函數(shù)對象).
在stackoverflow給出了類似與partial的運行方式
def partial(func, *part_args):
def wrapper(*extra_args):
args = list(part_args)
args.extend(extra_args)
return func(*args)
return wrapper
利用用閉包的特性綁定預(yù)先綁定一些函數(shù)參數(shù), 返回一個可調(diào)用的變量, 直到真正的調(diào)用執(zhí)行
#!/usr/bin/env python
# -*- coding: utf-8 -*-
from functools import partial
def sum(a, b):
return a + b
def test_partial():
fun = partial(sum, 2)? # 事先綁定一個參數(shù), fun成為一個只需要一個參數(shù)的可調(diào)用變量
print fun(3)? # 實現(xiàn)執(zhí)行的即是sum(2, 3)
if __name__ == '__main__':
test_partial()
# 執(zhí)行結(jié)果
5
eval我理解為一種內(nèi)嵌的python解釋器(這種解釋可能會有偏差), 會解釋字符串為對應(yīng)的代碼并執(zhí)行, 并且將執(zhí)行結(jié)果返回
看一下下面這個例子
#!/usr/bin/env python
# -*- coding: utf-8 -*-
def test_first():
return 3
def test_second(num):
return num
action = {? # 可以看做是一個sandbox
"para": 5,
"test_first" : test_first,
"test_second": test_second
}
def test_eavl():
condition = "para == 5 and test_second(test_first) > 5"
res = eval(condition, action)? # 解釋condition并根據(jù)action對應(yīng)的動作執(zhí)行
print res
if __name__ == '_
exec在Python中會忽略返回值, 總是返回None, eval會返回執(zhí)行代碼或語句的返回值
exec和eval在執(zhí)行代碼時, 除了返回值其他行為都相同
在傳入字符串時, 會使用compile(source, '', mode)編譯字節(jié)碼. mode的取值為exec和eval
#!/usr/bin/env python
# -*- coding: utf-8 -*-
def test_first():
print "hello"
def test_second():
test_first()
print "second"
def test_third():
print "third"
action = {
"test_second": test_second,
"test_third": test_third
}
def test_exec():
exec "test_second" in action
if __name__ == '__main__':
test_exec()? # 無法看到執(zhí)行結(jié)果
getattr(object, name[, default])Return the value of the named attribute of object. name must be a string. If the string is the name of one of the object’s attributes, the result is the value of that attribute. For example, getattr(x, ‘foobar’) is equivalent to x.foobar. If the named attribute does not exist, default is returned if provided, otherwise AttributeError is raised.
通過string類型的name, 返回對象的name屬性(方法)對應(yīng)的值, 如果屬性不存在, 則返回默認值, 相當于object.name
# 使用范例
class TestGetAttr(object):
test = "test attribute"
def say(self):
print "test method"
def test_getattr():
my_test = TestGetAttr()
try:
print getattr(my_test, "test")
except AttributeError:
print "Attribute Error!"
try:
getattr(my_test, "say")()
except AttributeError: # 沒有該屬性, 且沒有指定返回值的情況下
print "Method Error!"
if __name__ == '__main__':
test_getattr()
# 輸出結(jié)果
test attribute
test method
defprocess_command_line(argv):
"""
Return a 2-tuple: (settings object, args list).
`argv` is a list of arguments, or `None` for ``sys.argv[1:]``.
"""
ifargvisNone:
argv = sys.argv[1:]
# initialize the parser object:
parser = optparse.OptionParser(
formatter=optparse.TitledHelpFormatter(width=78),
add_help_option=None)
# define options here:
parser.add_option(# customized description; put --help last
'-h','--help', action='help',
help='Show this help message and exit.')
settings, args = parser.parse_args(argv)
# check number of arguments, verify values, etc.:
ifargs:
parser.error('program takes no command-line arguments; '
'"%s" ignored.'% (args,))
# further process settings & args if necessary
returnsettings, args
defmain(argv=None):
settings, args = process_command_line(argv)
# application code here, like:
# run(settings, args)
return0# success
if__name__ =='__main__':
status = main()
sys.exit(status)
# 從csv中讀取文件, 基本和傳統(tǒng)文件讀取類似
import csv
with open('data.csv', 'rb') as f:
reader = csv.reader(f)
for row in reader:
print row
# 向csv文件寫入
import csv
with open( 'data.csv', 'wb') as f:
writer = csv.writer(f)
writer.writerow(['name', 'address', 'age'])? # 單行寫入
data = [
( 'xiaoming ','china','10'),
( 'Lily', 'USA', '12')]
writer.writerows(data)? # 多行寫入
只發(fā)一張網(wǎng)上的圖, 然后差文檔就好了, 這個是記不住的
一個非常好用, 很多人又不知道的功能
>>> name = "andrew"
>>> "my name is {name}".format(name=name)
'my name is andrew'
# 還可以是?
"my name is {0}".format(name)
What is the difference between @staticmethod and @classmethod in Python?
Python @property versus getters and setters
How does the @property decorator work?
How does the functools partial work in Python?
What’s the difference between eval, exec, and compile in Python?
Be careful with exec and eval in Python
Python (and Python C API):newversusinit
Python ‘self’ keywordself不是關(guān)鍵字, 是一個約定的變量名
轉(zhuǎn)摘自:http://andrewliu.in/2015/11/14/Python%E5%A5%87%E6%8A%80%E6%B7%AB%E5%B7%A7/
作者在簡書的地址:http://www.reibang.com/u/4ee453b72aff
