Attribute Lookup: 當訪問實例的屬性時氓辣，發(fā)生了什么

原文地址, 拒絕轉載: http://www.reibang.com/p/b2691cf186d4

Attribute Lookup

假設 Cls 是類，instance 是類 Cls 的一個實例省有，當調用 instance.attr 時硬梁，到底發(fā)生了什么呢？下面就來一一探討屬性訪問的調用流程

1 descriptor

什么是 desciptor, 官方文檔給出的回答是

A descriptor is what we call any object that defines __get__(), __set__(), or __delete__().

即包含了任意 __get__ 或者 __set__ 或者 __delete__ 函數的方法的 object 都是 descriptor

2 data descriptor 與 non-data descriptors

If an object defines __set__() or __delete__(), it is considered a data descriptor. Descriptors that only define __get__() are called non-data descriptors (they are typically used for methods but other uses are possible).

如果一個 descriptor 只定義了 __get__ 方法淮野，那么就是 non-data descriptor

如果一個 object 定義了 __set__ 或者 __delete__ 方法，那么就是 data descriptor

class DataDescriptor:
    """
    包含了 __set__ 方法吹泡，所以這個類的實例是 data-descritptor
    """
    def __init__(self, init_value):
        self.value = init_value

    def __get__(self, instance, typ):
        return 'DataDescriptor __get__' + str(typ)

    def __set__(self, instance, value):
        print ('DataDescriptor __set__')
        self.value = value

class NonDataDescriptor:
    """
    只定義了 __get__ 方法骤星，所以這個類的實例是 non-data descriptor
    """
    def __init__(self, init_value):
        self.value = init_value

    def __get__(self, instance, typ):
        return'NonDataDescriptor __get__' + str(typ)

3 當調用 instance.attr 時，發(fā)生了什么

假設 cls 是類爆哑，instance 是類 cls 的一個實例洞难，當調用 instance.attr 時，調用流程如下

如果在cls 或者其基類中的 __dict__ 找到了 attr揭朝，并且 attr 是 data descriptor 則調用其 __get__方法队贱，即 __dict__['attr'].__get(instance, cls)

class Base(object):
    dd_base = DataDescriptor(0)
    ndd_base = NonDataDescriptor(0)

class Derive(Base):
    dd_derive = DataDescriptor(0)
    ndd_derive = NonDataDescriptor(0)
    ndd_derive2 = NonDataDescriptor(1)
    not_descriptor_in_class = "Derive not descriptor in class"
    
    def __getattr__(self, key):
        return '__getattr__ with key %s in Derive' % key

print(Base.__dict__)
"""
{
'__module__': '__main__', 
'dd_base': <__main__.DataDescriptor object at 0x7fc5c5b68a58>, 
'ndd_base': <__main__.NonDataDescriptor object at 0x7fc5c5b68a90>, 
'__dict__': <attribute '__dict__' of 'Base' objects>, 
'__weakref__': <attribute '__weakref__' of 'Base' objects>, 
'__doc__': None}
"""
print(Derive.__dict__)
"""
{'__module__': '__main__', 
'dd_derive': <__main__.DataDescriptor object at 0x7f9e74ac79b0>, 
'ndd_derive': <__main__.NonDataDescriptor object at 0x7f9e74ac79e8>, 'same_name_attr': 'attr in class', 
'__doc__': None}
"""
b = Base()
# 打印: DataDescriptor __get__<class '__main__.Base'>
print(b.dd_base)
d = Derive()
# 打印: DataDescriptor __get__<class '__main__.Derive'>
print(d.dd_base)
# 打印: DataDescriptor __get__<class '__main__.Derive'>
print(d.dd_derive)

# 即使我們更改了 instance 的 __dict__ 屬性，訪問時仍然從 data descriptor 中讀取
# 不會從 instance.__dict__ 中讀取
b.__dict__['dd_base'] = 'changed in dict dd base'
# 打印: DataDescriptor __get__<class '__main__.Base'>
print(b.dd_base)
d.__dict__['dd_derive'] = 'changed in dict dd derive'
# 打印: DataDescriptor __get__<class '__main__.Derive'>
print(d.dd_derive)

如果 attr 出現在 instance.__dict__ 中萝勤，則返回 instance.__dict__['attr']露筒。否則，執(zhí)行下面的流程

# 更改了 instance 的 __dict__
# 如果訪問的不是 data descriptor, 則直接中 instance.__dict__ 中讀取 attr
b.__dict__['ndd_base'] = 'changed in dict ndd base'
# 打印: changed in dict ndd base
print(b.ndd_base)
d.__dict__['ndd_derive'] = 'changed in dict ndd derive'
# 打印: changed in dict ndd derive
print(d.ndd_derive)

如果 attr 出現在類或者基類的 __dict__ 中

如果是 non-data descriptor, 則調用 __get__方法

# 打印: NonDataDescriptor __get__<class '__main__.Derive'>
print(d.ndd_derive2)

如果不是 descriptor , 則返回 __dict__['attr']

# 打印: Derive not descriptor in class
print(d.not_descriptor_in_class)

如果仍未找到敌卓，如果類或者其基類有 __getattr__ 方法慎式，則調用 __getattr__ 方法
```
# 打印: __getattr__ with key no_exist_key in Derive
print(d.no_exist_key)
```

否則拋出 AttributeError

try:
    b.no_exists_key
except Exception as e:
    # 打印: True
    print(isinstance(e, AttributeError))

4 完整測試代碼

class DataDescriptor:
    """
    包含了 __set__ 方法，所以這個類的實例是 data-descritptor
    """

    def __init__(self, init_value):
        self.value = init_value

    def __get__(self, instance, typ):
        return 'DataDescriptor __get__' + str(typ)

    def __set__(self, instance, value):
        print('DataDescriptor __set__')
        self.value = value


class NonDataDescriptor:
    """
    只定義了 __get__ 方法趟径，所以這個類的實例是 non-data descriptor
    """

    def __init__(self, init_value):
        self.value = init_value

    def __get__(self, instance, typ):
        return 'NonDataDescriptor __get__' + str(typ)


class Base(object):
    dd_base = DataDescriptor(0)
    ndd_base = NonDataDescriptor(0)


class Derive(Base):
    dd_derive = DataDescriptor(0)
    ndd_derive = NonDataDescriptor(0)
    ndd_derive2 = NonDataDescriptor(1)
    not_descriptor_in_class = "Derive not descriptor in class"

    def __getattr__(self, key):
        return '__getattr__ with key %s in Derive' % key


if __name__ == '__main__':
    b = Base()
    # 打印: DataDescriptor __get__<class '__main__.Base'>
    print(b.dd_base)
    d = Derive()
    # 打印: DataDescriptor __get__<class '__main__.Derive'>
    print(d.dd_base)
    # 打印: DataDescriptor __get__<class '__main__.Derive'>
    print(d.dd_derive)

    # 即使我們更改了 instance 的 __dict__ 屬性瘪吏，訪問時仍然從 data descriptor 中讀取
    # 不會從 instance.__dict__ 中讀取
    b.__dict__['dd_base'] = 'changed in dict dd base'
    # 打印: DataDescriptor __get__<class '__main__.Base'>
    print(b.dd_base)
    d.__dict__['dd_derive'] = 'changed in dict dd derive'
    # 打印: DataDescriptor __get__<class '__main__.Derive'>
    print(d.dd_derive)

    # 更改了 instance 的 __dict__
    # 如果訪問的不是 data descriptor, 則直接中 instance.__dict__ 中讀取 attr
    b.__dict__['ndd_base'] = 'changed in dict ndd base'
    # 打印: changed in dict ndd base
    print(b.ndd_base)
    d.__dict__['ndd_derive'] = 'changed in dict ndd derive'
    # 打印: changed in dict ndd derive
    print(d.ndd_derive)

    # 打印: NonDataDescriptor __get__<class '__main__.Derive'>
    print(d.ndd_derive2)
    # 打印: Derive not descriptor in class
    print(d.not_descriptor_in_class)

    # 打印: __getattr__ with key no_exist_key
    print(d.no_exist_key)

    try:
        b.no_exists_key
    except Exception as e:
        # 打印: True
        print(isinstance(e, AttributeError))

5 參考鏈接

https://blog.peterlamut.com/2018/11/04/python-attribute-lookup-explained-in-detail/
https://docs.python.org/3/howto/descriptor.html
https://www.cnblogs.com/xybaby/p/6270551.html

6 轉載一下調用流程圖片

圖片是從參考鏈接的第一個博客中復制的，把調用的流程描述的很清晰蜗巧，值得一看

Attribute Lookup: 當訪問實例的屬性時，發(fā)生了什么

Attribute Lookup: 當訪問實例的屬性時棋枕，發(fā)生了什么