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如何在不破坏默认行为的情况下在Python中重写__getattr__？

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问题：如何在不破坏默认行为的情况下在Python中重写__getattr__？
回答 0
回答 1
回答 2

问题：如何在不破坏默认行为的情况下在Python中重写__getattr__？

我想重写__getattr__类上的方法以执行某些操作，但我不想破坏默认行为。

正确的方法是什么？

点击查看英文原文

I want to override the __getattr__ method on a class to do something fancy but I don’t want to break the default behavior.

What’s the correct way to do this?

回答 0

覆盖__getattr__应该没问题- __getattr__仅可作为最后的选择，即，如果实例中没有与名称匹配的属性。例如，如果您访问foo.bar，则__getattr__仅当foo没有调用属性时才会被调用bar。如果该属性是您不想处理的属性，请引发AttributeError：

class Foo(object):
    def __getattr__(self, name):
        if some_predicate(name):
            # ...
        else:
            # Default behaviour
            raise AttributeError

但是，与不同的是__getattr__，__getattribute__将首先调用（仅适用于新样式类，即从对象继承的类）。在这种情况下，您可以保留默认行为，如下所示：

class Foo(object):
    def __getattribute__(self, name):
        if some_predicate(name):
            # ...
        else:
            # Default behaviour
            return object.__getattribute__(self, name)

有关更多信息，请参见Python文档。

点击查看英文原文

Overriding __getattr__ should be fine — __getattr__ is only called as a last resort i.e. if there are no attributes in the instance that match the name. For instance, if you access foo.bar, then __getattr__ will only be called if foo has no attribute called bar. If the attribute is one you don’t want to handle, raise AttributeError:

class Foo(object):
    def __getattr__(self, name):
        if some_predicate(name):
            # ...
        else:
            # Default behaviour
            raise AttributeError

However, unlike __getattr__, __getattribute__ will be called first (only works for new style classes i.e. those that inherit from object). In this case, you can preserve default behaviour like so:

class Foo(object):
    def __getattribute__(self, name):
        if some_predicate(name):
            # ...
        else:
            # Default behaviour
            return object.__getattribute__(self, name)

See the Python docs for more.

回答 1

class A(object):
    def __init__(self):
        self.a = 42

    def __getattr__(self, attr):
        if attr in ["b", "c"]:
            return 42
        raise AttributeError("%r object has no attribute %r" %
                             (self.__class__.__name__, attr))

>>> a = A()
>>> a.a
42
>>> a.b
42
>>> a.missing
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "<stdin>", line 8, in __getattr__
AttributeError: 'A' object has no attribute 'missing'
>>> hasattr(a, "b")
True
>>> hasattr(a, "missing")
False
点击查看英文原文 
class A(object):
    def __init__(self):
        self.a = 42

    def __getattr__(self, attr):
        if attr in ["b", "c"]:
            return 42
        raise AttributeError("%r object has no attribute %r" %
                             (self.__class__.__name__, attr))

>>> a = A()
>>> a.a
42
>>> a.b
42
>>> a.missing
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "<stdin>", line 8, in __getattr__
AttributeError: 'A' object has no attribute 'missing'
>>> hasattr(a, "b")
True
>>> hasattr(a, "missing")
False

回答 2

为了扩展Michael的答案，如果您想使用来维持默认行为__getattr__，可以这样做： 

class Foo(object):
    def __getattr__(self, name):
        if name == 'something':
            return 42

        # Default behaviour
        return self.__getattribute__(name)

现在，异常消息更具描述性：

>>> foo.something
42
>>> foo.error
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "<stdin>", line 5, in __getattr__
AttributeError: 'Foo' object has no attribute 'error'
点击查看英文原文

To extend Michael answer, if you want to maintain the default behavior using __getattr__, you can do it like so: 

class Foo(object):
    def __getattr__(self, name):
        if name == 'something':
            return 42

        # Default behaviour
        return self.__getattribute__(name)

Now the exception message is more descriptive:

>>> foo.something
42
>>> foo.error
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "<stdin>", line 5, in __getattr__
AttributeError: 'Foo' object has no attribute 'error'