Python Cookbook Third Edition learning Note 12: Classes and Objects (iii) Create a new class or instance property

The application __getattr__,__setattr__,__get__,__set__,__getattribute__ in several classes are introduced first . 

__getattr__: When attribute is not found in the class , __getattr__ is called and the custom code is executed. All the properties defined in the class are contained in __dict__, that is, __getattr__ is triggered if the corresponding property name is not found in the __dict__ . 

classGet_try (Object):
def__init__(Self, value):
 Self. value=value
def__getattr__ (self         self< Span lang= "en-US" >.value=item 
 
  if   __name__ = =   "__main__"   : 
     g=get_try (  ' value '        g.value1   #调用了 g.value1, Value1 incoming __getattr__ item. After calling __getattr, value=value1 
      print   g.value              

G.__dict_

E:\python2.7.11\python.exe e:/py_prj/fluent_python/chapter7.py

Value1

{' Value ': ' value1 '}

The g.value value is value1  When the result is printed

__GETATTRIBUTE__: Called unconditionally,  __getattr__ is not called if __getattr__ is defined at the same time. 

classGet_try (Object):
def__init__(Self, value):
Self.value=value 
      def   __getattr__ (self          self.value=item 
       def   __getattribute__ ( Span lang= "en-US" >self          print   item 
 
  if   Span lang= "en-us" >__name__ = =   "__main__"   : 
      g=get_try (  ' value '   ) 
      g.value1              

E:\python2.7.11\python.exe e:/py_prj/fluent_python/chapter7.py

Value1

The result is the same as before.

__setattr__ is called when it is necessary to define the property itself. For example Self.att=value will become self.__setattr__ ("att", value)get_try ():   __init__(self, value):   

Self. value=value
def__getattr__(Self, item):
Self. Value=item
def__getattribute__(Self, item):
Printitem 
      def    __setattr__ (self          self.__dict__[key]=value 
 
 
  Span lang= "en-US" >if   __name__ = =   "__main__"   : 
     g=get_try (  ' value '        g.value1=3 
      print   g.value1 
      print   g.__dict__             

E:\python2.7.11\python.exe e:/py_prj/python_cookbook/chapter8.py

3

{' value1 ': 3, ' value ': ' Value '}

It is important to note that the Self.key=value assignment cannot be performed at the time of __setattr__ , because this means that the self.__setattr__ is not kept in call . This creates an infinite recursive loop. Eventually causes a stack overflow exception. When we call, the error of the report is like this  

File "e:/py_prj/python_cookbook/chapter8.py", line 107, in __setattr__

Self.key=value

File "e:/py_prj/python_cookbook/chapter8.py", line 107, in __setattr__

Self.key=value

Runtimeerror:maximum recursion depth exceeded

In fact, when we do not implement __setattr__, we also implicitly call this function when assigning a value to a property. So what are the benefits of this implementation? The advantage is that we can assign to which properties to assign values. 

classGet_try ():
def__init__(Self, value):
Self. value=value
def__getattr__(Self, item):
Self. Value=item
def__getattribute__(Self, item):
PrintItem
def__setattr__(Self, key, value):
ifKey = =' value1 ':
 print ' incorrect key '
else   :
self.  __dict__[Key]=value


if __name__ = = "__main__":
G=get_try (' value ')
g.value1=3
g.value2=4
Print G.__dict__            

E:\python2.7.11\python.exe e:/py_prj/python_cookbook/chapter8.py

Incorrect key

{' value2 ': 4, ' value ': ' Value '}

In the above code, when assigning a value to a value1, it is first judged. If it is value1, print incorrect key. So it's a success to see when the value2 is assigned. In the same way, we can use __setattr__ to prevent subsequent assignments of properties that are already in the class:    

classGet_try ():
def__init__(Self, value):
Self. value=value
def__getattr__(Self, item):
 Self. Value=item
def__getattribute__(Self, item):
PrintItem
def__setattr__(Self, key, value):
offKeyinchSelf.__dict__:
  Print ' already exist '
else   :
self.  __dict__[Key]=value

if __name__ = = "__main__":
G=get_try (' value ')
G.value=4
g.value1=3
Print G.__dict__            

E:\python2.7.11\python.exe e:/py_prj/python_cookbook/chapter8.py

Already exist

{' value1 ': 3, ' value ': ' Value '}

The above code first determines whether the assigned object already exists and prints already if it exists exist

Python Cookbook Third Edition learning Note 12: Classes and Objects (iii) Create a new class or instance property