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顯示有限的接口到外部
當發佈python第三方package時, 並不希望代碼中所有的函數或者class可以被外部import, 在__init__.py
中添加__all__
屬性,
該list中填寫可以import的類或者函數名, 可以起到限制的import的作用, 防止外部import其他函數或者類
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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']
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with的魔力
with語句需要支持上下文管理協議的對象
,
上下文管理協議包含__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行進入和退出
操作.
其中上下文表達式
是跟在with之後的表達式,
該表示大返回一個上下文管理對象
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with open("test.txt", "r") as my_file: for line in my_file: print line
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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議的類, 類中實現__enter__
和__exit__
方法
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class MyWith(object): def __init__(self): print "__init__ method" def __enter__(self): print "__enter__ method" return self 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()
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執行結果如下:
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__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 <module> exec(compile(__file__f.read(), __file__, "exec")) File "test_with.py", line 33, in <module> test_with() File "test_with.py", line 28, in test_with raise Exception Exception
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證明了會先執行__enter__
方法,
然後調用with內的邏輯, 最後執行__exit__
做退出處理,
並且, 即使出現異常也能正常退出
filter的用法
相對filter
而言,
map和reduce使用的會更頻繁一些, filter
正如其名字,
按照某種規則過濾
掉一些元素
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#!/usr/bin/env python # -*- coding: utf-8 -*- lst = [1, 2, 3, 4, 5, 6] # 所有奇數都會返回True, 偶數會返回False被過濾掉 print filter(lambda x: x % 2 != 0, lst) #輸出結果 [1, 3, 5]
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一行作判斷
當條件滿足時, 返回的爲等號後面的變量, 否則返回else後語句
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lst = [1, 2, 3] new_lst = lst[0] if lst is not None else None print new_lst 1
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裝飾器之單例
使用裝飾器實現簡單的單例模式
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def singleton(cls): instances = dict() def _singleton(*args, **kwargs): if cls not in instances: instances[cls] = cls(*args, **kwargs) return instances[cls] return _singleton @singleton class Test(object): pass if __name__ == '__main__': t1 = Test() t2 = Test() print t1, t2
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staticmethod裝飾器
類中兩種常用的裝飾, 首先區分一下他們
-
普通成員函數, 其中第一個隱式參數爲
對象
-
classmethod裝飾器
,
類方法(給人感覺非常類似於OC中的類方法), 其中第一個隱式參數爲類
-
staticmethod裝飾器
,
沒有任何隱式參數. python中的靜態方法類似與C++中的靜態方法
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class A(object): def foo(self, x): print "executing foo(%s, %s)" % (self, x) @classmethod def class_foo(cls, x): print "executing class_foo(%s, %s)" % (cls, x) @staticmethod def static_foo(x): print "executing static_foo(%s)" % x def test_three_method(): obj = A() obj.foo("para") obj.class_foo("para") A.class_foo("para") obj.static_foo("para") A.static_foo("para") if __name__ == '__main__': test_three_method() executing foo(<__main__.A object at 0x100ba4e10>, para) executing class_foo(<class '__main__.A'>, para) executing class_foo(<class '__main__.A'>, para) executing static_foo(para) executing static_foo(para)
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property裝飾器
將property
與裝飾器結合實現屬性私有化(更簡單安全的實現get和set方法
)
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property(fget=None, fset=None, fdel=None, doc=None)
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fget
是獲取屬性的值的函數,fset
是設置屬性值的函數,fdel
是刪除屬性的函數,doc
是一個字符串(like
a comment).從實現來看,這些參數都是可選的
property有三個方法getter()
, setter()
和delete()
來指定fget,
fset和fdel。 這表示以下這行
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class Student(object): @property #相當於property.getter(score) 或者property(score) def score(self): return self._score @score.setter #相當於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
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iter魔法
-
通過yield和
__iter__
的結合,
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
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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()
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神奇partial
partial使用上很像C++中仿函數(函數對象).
在stackoverflow給出了類似與partial的運行方式
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def partial(func, *part_args): def wrapper(*extra_args): args = list(part_args) args.extend(extra_args) return func(*args) return wrapper
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利用用閉包的特性綁定預先綁定一些函數參數, 返回一個可調用的變量, 直到真正的調用執行
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from functools import partial def sum(a, b): return a + b def test_partial(): fun = partial(sum, 2) print fun(3) if __name__ == '__main__': test_partial() 5
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神祕eval
eval我理解爲一種內嵌的python解釋器(這種解釋可能會有偏差), 會解釋字符串爲對應的代碼並執行, 並且將執行結果返回
看一下下面這個例子
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def test_first(): return 3 def test_second(num): return num action = { "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) print res if __name__ == '_
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exec
-
exec在Python中會忽略返回值, 總是返回None, eval會返回執行代碼或語句的返回值
-
exec
和eval
在執行代碼時,
除了返回值其他行爲都相同 -
在傳入字符串時, 會使用
compile(source,
'<string>', mode)
編譯字節碼. mode的取值爲exec
和eval
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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()
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getattr
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)對應的值, 如果屬性不存在, 則返回默認值, 相當於object.name
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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: print "Method Error!" if __name__ == '__main__': test_getattr() test attribute test method
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命令行處理
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def process_command_line(argv): """ Return a 2-tuple: (settings object, args list). `argv` is a list of arguments, or `None` for ``sys.argv[1:]``. """ if argv is None: argv = sys.argv[1:] parser = optparse.OptionParser( formatter=optparse.TitledHelpFormatter(width=78), add_help_option=None) parser.add_option( '-h', '--help', action='help', help='Show this help message and exit.') settings, args = parser.parse_args(argv) if args: parser.error('program takes no command-line arguments; ' '"%s" ignored.' % (args,)) return settings, args def main(argv=None): settings, args = process_command_line(argv) return 0 if __name__ == '__main__': status = main() sys.exit(status)
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讀寫csv文件
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import csv with open('data.csv', 'rb') as f: reader = csv.reader(f) for row in reader: print row import csv with open( 'data.csv', 'wb') as f: writer = csv.writer(f) writer.writerow(['name', 'address', 'age']) data = [ ( 'xiaoming ','china','10'), ( 'Lily', 'USA', '12')] writer.writerows(data)
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各種時間形式轉換
只發一張網上的圖, 然後差文檔就好了, 這個是記不住的
字符串格式化
一個非常好用, 很多人又不知道的功能
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>>> name = "andrew" >>> "my name is {name}".format(name=name) 'my name is andrew'
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參考鏈接