python技巧31[python Tip2]

1 函数的默认值为mutable类型时的问题和解决办法

def f2(a, L=[]):
    L.append(a)
    return L
print(f2(1))
print(f2(2))
print(f2(3))
def f3(a, L=None):
    if L is None:
        L = []
    L.append(a)
    return L
print(f3(1))
print(f3(2))
print(f3(3))
# the result will be
#[1]
#[1, 2]
#[1, 2, 3]
#[1]
#[2]
#[3]

 

from copy import deepcopy
def resetDefaults(f):
    defaults = f.__defaults__
    def resetter(*args, **kwds):
        f.__defaults__ = deepcopy(defaults)
        return f(*args, **kwds)
                    
    resetter.__name__ = f.__name__
    return resetter
                            
@resetDefaults # This is how you apply a decorator 
def TestDefaultCorrect(item, stuff = []): 
     stuff.append(item) 
     print (stuff)

TestDefaultCorrect(1)
# prints '[1]' 
TestDefaultCorrect(2)
# prints '[2]', as expected

2 函数装饰模式

def decorator1(func):
    return lambda: func() + 1

def decorator2(func):
    def print_func(): 
        print (func())
    return print_func

@decorator2
@decorator1
def function():
    return 41

# to cal functions(), it is equal to call decorator2(decorator1(function))

function()
# prints '42'

3 检查类型的属性和方法是否存在

class Class:
    answer = 42

getattr(Class, 'answer')
# returns 42
getattr(Class, 'question', 'What is six times nine?')
# returns 'What is six times nine?'
getattr(Class, 'question')
# raises AttributeError

4 动态修改类中的函数

class Class: 
   def method(self): 
       print ('Hey a method' )
       
instance = Class() 
instance.method() 
# prints 'Hey a method' 

def new_method(self):
    print ('New method wins!')
    
Class.method = new_method
instance.method()
# prints 'New method wins!'

5 类的静态方法的使用

class Class:
    @classmethod
    def a_class_method(cls): 
       print ('I was called from class %s' % cls)

    @staticmethod
    def a_static_method(): 
        print ('I have no idea where I was called from')
        
    def another_static_method():
         print ('I have no idea where I was called from2')

    def an_instance_method(self):
        print ('I was called from the instance %s' % self)

instance = Class()

Class.a_class_method()
instance.a_class_method()
# both print 'I was called from class __main__.Class'

Class.a_static_method()
instance.a_static_method()
# both print 'I have no idea where I was called from'

Class.another_static_method()
# both print 'I have no idea where I was called from2'
#instance.another_static_method()
#TypeError: another_static_method() takes no arguments (1 given)

#Class.an_instance_method()
# TypeError: an_instance_method() takes exactly 1 positional argument (0 given)
instance.an_instance_method()
# prints something like 'I was called from the instance <__main__.Class instance at 0x2e80d0>'

6 使用main来作为python文件的入口

if __name__ == "__main__":

7 将函数的输出重定向到文件 （以下代码有个bug）

import sys

def stdoutToFile(filename, function, args ):
    oldStdout = sys.stdout
    f = open(filename, "w" )
    sys.stdout = f 
    function(args)
    #sys.stdout.flush()
    #f.close()
    sys.stdout = oldStdout


if __name__=='__main__':
  print("modules")
  stdoutToFile("modules.txt", help, "modules")
  print("builtins")
  stdoutToFile("builtins.txt", help, "builtins")
  print("keywords")
  stdoutToFile("keyword.txt", help, "keywords")

参考：http://www.siafoo.net/article/52#id26

完!