python - 实现了自己的 __getattr__；遇到意外错误

Question

这是我的简化类(有一个错误):

$ ipython

Python 2.7.6 (default, Oct 26 2016, 20:30:19)
Type "copyright", "credits" or "license" for more information.

IPython 5.1.0 -- An enhanced Interactive Python.
?         -> Introduction and overview of IPython's features.
%quickref -> Quick reference.
help      -> Python's own help system.
object?   -> Details about 'object', use 'object??' for extra details.


In [1]: import numpy as np

In [2]: class Foo(object):
   ...:     def __init__(self):
   ...:         self.one = 1
   ...:         self.dct = dict(a='aaa', b='bbb')
   ...:
   ...:     @property
   ...:     def two(self):
   ...:         # import ipdb; ipdb.set_trace()
   ...:         np.ann  # This was the spelling error I had
   ...:         return 2
   ...:
   ...:     def __getattr__(self, key):
   ...:         """
   ...:         Provide convenient access to values that are
   ...:         somewhat inconvient to access, eg
   ...:         
   ...:             >>> foo = Foo()
   ...:             >>> foo.dct['a']  # this obviously works
   ...:             'aaa'
   ...:             >>> foo.a  # but this is easier
   ...:             'aaa'
   ...:         
   ...:         In reality I have something a bit more complicated than
   ...:         a simple dictionary (`self.dct = dict(...)`)
   ...:         
   ...:         """
   ...:         print('__getattr__ with "{}"'.format(key))
   ...:         try:
   ...:             return self.dct[key]
   ...:         except KeyError:
   ...:             raise AttributeError("Can't find '{}'".format(key))
   ...:

使用它...

In [3]: foo = Foo()

In [4]: foo.one
Out[4]: 1

In [5]: foo.dct['a']
Out[5]: 'aaa'

In [6]: foo.a
__getattr__ with "a"
Out[6]: 'aaa'

In [7]: foo.two
__getattr__ with "two"
---------------------------------------------------------------------------
AttributeError                            Traceback (most recent call last)
<ipython-input-9-29e77587894c> in <module>()
----> 1 foo.two

<ipython-input-4-955e08b9c001> in __getattr__(self, key)
     12             return self.dct[key]
     13         except KeyError:
---> 14             raise AttributeError("Can't find '{}'".format(key))
     15

AttributeError: Can't find 'two'

为什么拼写错误 np.ann 调用 foo.__getattr__ 并在我自己的 raise AttributeError 中结束 key = = '二'??

我已经逐步完成了代码(通过取消注释 set_trace() 行)，但并没有真正理解它。如果有帮助，这里是输出。

In [15]: foo.two
> <ipython-input-13-915f93b88a22>(8)two()
      7         import ipdb; ipdb.set_trace()
----> 8         np.ann
      9         return 2

ipdb> n
AttributeError: "'module' object has no attribute 'ann'"
> <ipython-input-13-915f93b88a22>(8)two()
      7         import ipdb; ipdb.set_trace()
----> 8         np.ann
      9         return 2

ipdb> n
--Return--
None
> <ipython-input-13-915f93b88a22>(8)two()
      7         import ipdb; ipdb.set_trace()
----> 8         np.ann
      9         return 2

ipdb> n
--Call--
> <ipython-input-13-915f93b88a22>(10)__getattr__()
      9         return 2
---> 10     def __getattr__(self, key):
     11         print('__getattr__ with "{}"'.format(key))

ipdb> n
> <ipython-input-13-915f93b88a22>(11)__getattr__()
     10     def __getattr__(self, key):
---> 11         print('__getattr__ with "{}"'.format(key))
     12         try:

ipdb> n
__getattr__ with "two"
> <ipython-input-13-915f93b88a22>(12)__getattr__()
     11         print('__getattr__ with "{}"'.format(key))
---> 12         try:
     13             return self.dct[key]

ipdb> n
> <ipython-input-13-915f93b88a22>(13)__getattr__()
     12         try:
---> 13             return self.dct[key]
     14         except KeyError:

ipdb> n
KeyError: ('two',)
> <ipython-input-13-915f93b88a22>(13)__getattr__()
     12         try:
---> 13             return self.dct[key]
     14         except KeyError:

ipdb> n
> <ipython-input-13-915f93b88a22>(14)__getattr__()
     13             return self.dct[key]
---> 14         except KeyError:
     15             raise AttributeError("Can't find '{}'".format(key))

ipdb> n
> <ipython-input-13-915f93b88a22>(15)__getattr__()
     14         except KeyError:
---> 15             raise AttributeError("Can't find '{}'".format(key))
     16

ipdb> n
AttributeError: Attribut... 'two'",)
> <ipython-input-13-915f93b88a22>(15)__getattr__()
     14         except KeyError:
---> 15             raise AttributeError("Can't find '{}'".format(key))
     16

ipdb> n
--Return--
None
> <ipython-input-13-915f93b88a22>(15)__getattr__()
     14         except KeyError:
---> 15             raise AttributeError("Can't find '{}'".format(key))
     16

ipdb> n
AttributeError: Attribut... 'two'",)
> <ipython-input-15-29e77587894c>(1)<module>()
----> 1 foo.two

ipdb> n
--Return--
None
> <ipython-input-15-29e77587894c>(1)<module>()
----> 1 foo.two

ipdb> n
AttributeError: Attribut... 'two'",)
> /home/venv/local/lib/python2.7/site-packages/IPython/core/interactiveshell.py(2881)run_code()
   2880                 #rprint('Running code', repr(code_obj)) # dbg
-> 2881                 exec(code_obj, self.user_global_ns, self.user_ns)
   2882             finally:

如果我注释掉 @property 行，那么这个 bug 就会按预期运行，这让我相信这与装饰器有关...:

In [19]: foo.two
Out[19]: <bound method Foo.two of <__main__.Foo object at 0x7fc248156450>>

In [20]: foo.two()
---------------------------------------------------------------------------
AttributeError                            Traceback (most recent call last)
<ipython-input-20-25d037a9762c> in <module>()
----> 1 foo.two()

<ipython-input-16-9999eb93c349> in two(self)
      6     def two(self):
      7         # import ipdb; ipdb.set_trace()
----> 8         np.ann
      9         return 2
     10     def __getattr__(self, key):

AttributeError: 'module' object has no attribute 'ann'

Answer 1

• (正常行为)每当正常属性查找引发 AttributeError 时调用 __getattr__。

• property 更改属性查找，以便在您访问属性时执行函数。

在您的情况下，您想要访问 two 属性，该属性又会调用方法 Foo.two，该方法在以下情况下引发 AttributeError它尝试查找 np.ann。此 AttributeError 被捕获，您的 __getattr__ 被调用 key='two'。但是因为 'two' 不在你的字典中，你会得到你在那里提出的 AttributeError。