Python對象的屬性訪問過程詳解

class A(object):
  a = 'a'

class B(A):
  b = 'b'

class C(B):
  class_field = 'class field'
  def __getattr__(self, f):
    print('Method {}.__getattr__ has been called.'.format(
      self.__class__.__name__))
    return f
c = C()
print c.a
print c.b
print c.class_field
print c.c

a
b
class field
Method C.__getattr__ has been called.
c

>>> class Test(object):
    pass
>>> test = Test()
# 查看類屬性
>>> dir(Test)
['__class__','__delattr__','__dict__','__doc__','__format__',
'__getattribute__', '__hash__', '__init__', '__module__',
 '__new__', '__reduce__', '__reduce_ex__', '__repr__', 
 '__setattr__', '__sizeof__', '__str__', '__subclasshook__',
 '__weakref__']
# 查看實例屬性
>>> dir(test)
['__class__', '__delattr__', '__dict__', '__doc__', '__format__', 
'__getattribute__', '__hash__', '__init__', '__module__',
 '__new__', '__reduce__', '__reduce_ex__', '__repr__',
 '__setattr__', '__sizeof__', '__str__', '__subclasshook__', 
 '__weakref__']

>>> class Spring(object):
...   season = "the spring of class"
... 

# 查看Spring類保存的屬性
>>> Spring.__dict__
dict_proxy({'__dict__': <attribute '__dict__' of 'Spring' objects>, 
'season': 'the spring of class', 
'__module__': '__main__', 
'__weakref__': <attribute '__weakref__' of 'Spring' objects>, 
'__doc__': None})

# 通過兩種方法訪問類屬性
>>> Spring.__dict__['season']
'the spring of class'
>>> Spring.season
'the spring of class'

>>> s = Spring()
# 實例屬性的__dict__是空的
>>> s.__dict__
{}
# 其實是指向的類屬性
>>> s.season
'the spring of class'

# 建立實例屬性
>>> s.season = "the spring of instance"
# 這樣，實例屬性里面就不空了。這時候建立的實例屬性和類屬性重名，并且把它覆蓋了
>>> s.__dict__
{'season': 'the spring of instance'}
>>> s.__dict__['season']
'the spring of instance'
>>> s.season
'the spring of instance'

# 類屬性沒有受到實例屬性的影響
>>> Spring.__dict__['season']
'the spring of class'
>>> Spring.__dict__
dict_proxy({'__dict__': <attribute '__dict__' of 'Spring' objects>, 'season': 'the spring of class', '__module__': '__main__', '__weakref__': <attribute '__weakref__' of 'Spring' objects>, '__doc__': None})

# 如果將實例屬性刪除,又會調用類屬性
>>> del s.season
>>> s.__dict__
{}
>>> s.season
'the spring of class'

# 自定義實例屬性，對類屬性沒有影響
>>> s.lang = "python"
>>> s.__dict__
{'lang': 'python'}
>>> s.__dict__['lang']
'python'

# 修改類屬性
>>> Spring.flower = "peach"
>>> Spring.__dict__
dict_proxy({'__module__': '__main__', 
'flower': 'peach', 
'season': 'the spring of class', 
'__dict__': <attribute '__dict__' of 'Spring' objects>, '__weakref__': <attribute '__weakref__' of 'Spring' objects>, '__doc__': None})
>>> Spring.__dict__['flower']
'peach'
# 實例中的__dict__并沒有變化
>>> s.__dict__
{'lang': 'python'}
# 實例中找不到flower屬性，調用類屬性
>>> s.flower
'peach'

# 定義類
>>> class Spring(object):
...   def tree(self, x):
...     self.x = x
...     return self.x
... 
# 方法tree在__dict__里面
>>> Spring.__dict__
dict_proxy({'__dict__': <attribute '__dict__' of 'Spring' objects>, 
'__weakref__': <attribute '__weakref__' of 'Spring' objects>, 
'__module__': '__main__', 
'tree': <function tree at 0xb748fdf4>, 
'__doc__': None})
>>> Spring.__dict__['tree']
<function tree at 0xb748fdf4>

# 建立實例，但是__dict__中沒有方法 
>>> t = Spring()
>>> t.__dict__
{}

# 執(zhí)行方法
>>> t.tree("xiangzhangshu")
'xiangzhangshu'
# 實例方法(t.tree('xiangzhangshu'))的第一個參數(shù)(self，但沒有寫出來)綁定實例 t，透過 self.x 來設定值，即給 t.__dict__添加屬性值。
>>> t.__dict__
{'x': 'xiangzhangshu'}
# 如果沒有將x 賦值給 self 的屬性，而是直接 return，結果發(fā)生了變化
>>> class Spring(object):
...   def tree(self, x):
...     return x
>>> s = Spring()
>>> s.tree("liushu")
'liushu'
>>> s.__dict__
{}

# 在類中定義__slots__屬性就是說這個類中所有實例的屬性都在這兒了，如果幾百萬個實例同時活動，能節(jié)省大量內存
>>> class Spring(object):
...   __slots__ = ("tree", "flower")
... 
# 仔細看看 dir() 的結果，還有__dict__屬性嗎？沒有了，的確沒有了。也就是說__slots__把__dict__擠出去了，它進入了類的屬性。
>>> dir(Spring)
['__class__', '__delattr__', '__doc__', '__format__', '__getattribute__', '__hash__', '__init__', '__module__', '__new__', '__reduce__', '__reduce_ex__', '__repr__', '__setattr__', '__sizeof__', '__slots__', '__str__', '__subclasshook__', 'flower', 'tree']
>>> Spring.__slots__
('tree', 'flower')
# 實例化
>>> t = Spring()
>>> t.__slots__
('tree', 'flower')

# 通過類賦予屬性值
>>> Spring.tree = "liushu"
# tree這個屬性是只讀的, 實例不能修改
>>> t.tree = "guangyulan"
Traceback (most recent call last):
 File "<stdin>", line 1, in <module>
AttributeError: 'Spring' object attribute 'tree' is read-only
>>> t.tree
'liushu'

# 對于用類屬性賦值的屬性，只能用來修改
>>> Spring.tree = "guangyulan"
>>> t.tree
'guangyulan'

# 對于沒有用類屬性賦值的屬性，可以通過實例來修改
>>> t.flower = "haitanghua"
>>> t.flower
'haitanghua'
# 實例屬性的值并沒有傳回到類屬性，你也可以理解為新建立了一個同名的實例屬性
>>> Spring.flower
<member 'flower' of 'Spring' objects>
# 如果再給類屬性賦值
>>> Spring.flower = "ziteng"
>>> t.flower
'ziteng'

__setattr__(self,name,value)：如果要給 name 賦值，就調用這個方法。
__getattr__(self,name)：如果 name 被訪問，同時它不存在的時候，此方法被調用。
__getattribute__(self,name)：當 name被訪問時自動被調用（注意：這個僅能用于新式類），無論 name 是否存在，都要被調用。
__delattr__(self,name)：如果要刪除 name，這個方法就被調用。
>>> class A(object):
...   def __getattr__(self, name):
...     print "You use getattr"
...   def __setattr__(self, name, value):
...     print "You use setattr"
...     self.__dict__[name] = value
# a.x，按照本節(jié)開頭的例子，是要報錯的。但是，由于在這里使用了__getattr__(self, name) 方法，當發(fā)現(xiàn) x 不存在于對象的__dict__中的時候，就調用了__getattr__，即所謂“攔截成員”。
>>> a = A()
>>> a.x
You use getattr

# 給對象的屬性賦值時候，調用了__setattr__(self, name, value)方法，這個方法中有一句 self.__dict__[name] = value，通過這個語句，就將屬性和數(shù)據保存到了對象的__dict__中
>>> a.x = 7
You use setattr

# 測試__getattribute__(self,name)
>>> class B(object):
...   def __getattribute__(self, name):
...     print "you are useing getattribute"
...     return object.__getattribute__(self, name)
# 返回的內容用的是 return object.__getattribute__(self, name)，而沒有使用 return self.__dict__[name]。因為如果用這樣的方式，就是訪問 self.__dict__，只要訪問這個屬性，就要調用`getattribute``，這樣就導致了無限遞歸

# 訪問不存在的成員，可以看到，已經被__getattribute__攔截了，雖然最后還是要報錯的。
>>> b = B()
>>> b.y
you are useing getattribute
Traceback (most recent call last):
 File "<stdin>", line 1, in <module>
 File "<stdin>", line 4, in __getattribute__
AttributeError: 'B' object has no attribute 'y'

class Vector(object):
  def __init__(self, x, y):
    # 使用兩個前導下劃線，把屬性標記為私有
    self.__x = float(x)
    self.__y = float(y)
  
  # porperty裝飾器把讀值方法標記為特性
  @property
  def x(self):
    return self.__x
    
  @property
  def y(self):
    return self.__y
    
vector = Vector(3,4)
print(vector.x, vector.y)

class Rectangle(object):
  """
  the width and length of Rectangle
  """
  def __init__(self):
    self.width = 0
    self.length = 0

  def setSize(self, size):
    self.width, self.length = size
  def getSize(self):
    return self.width, self.length

if __name__ == "__main__":
  r = Rectangle()
  r.width = 3
  r.length = 4
  print r.getSize()  # (3,4)
  r.setSize( (30, 40) )
  print r.width  # 30
  print r.length  # 40

class Rectangle(object):
  """
  the width and length of Rectangle
  """
  def __init__(self):
    self.width = 0
    self.length = 0

  def setSize(self, size):
    self.width, self.length = size
  def getSize(self):
    return self.width, self.length
  # 使用property方法將函數(shù)封裝為屬性，更優(yōu)雅
  size = property(getSize, setSize)

if __name__ == "__main__":
  r = Rectangle()
  r.width = 3
  r.length = 4
  print r.size   # (30, 40)
  r.size = 30, 40
  print r.width  # 30
  print r.length  # 40

class NewRectangle(object):
  def __init__(self):
    self.width = 0
    self.length = 0
  
  def __setattr__(self, name, value):
    if name == 'size':
      self.width, self, length = value
    else:
      self.__dict__[name] = value
      
  def __getattr__(self, name):
    if name == 'size':
      return self.width, self.length
    else:
      raise AttrubuteErrir
      
if __name__ == "__main__":
  r = Rectangle()
  r.width = 3
  r.length = 4
  print r.size   # (30, 40)
  r.size = 30, 40
  print r.width  # 30
  print r.length  # 40

class A(object):
  author = "qiwsir"
  def __getattr__(self, name):
    if name != "author":
      return "from starter to master."

if __name__ == "__main__":
  a = A()
  print a.author # qiwsir
  print a.lang # from starter to master.