Why doesn't __getattr__ work with __exit__?

Question

I came across this as a bit of a surprise while trying to work out another question.

This seemed extremely odd to me, I thought it was worth asking the question. Why doesn't __getattr__ appear to work with with?

if I make this object:

class FileHolder(object):
    def __init__(self,*args,**kwargs):
        self.f= file(*args,**kwargs)

    def __getattr__(self,item):
        return getattr(self.f,item)

and using it with with,

>>> a= FileHolder("a","w")
>>> a.write
<built-in method write of file object at 0x018D75F8>
>>> with a as f:
...   print f
...
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
AttributeError: __exit__
>>> a.__exit__
<built-in method __exit__ of file object at 0x018D75F8>

Why does this happen?

EDIT

>>> object.__exit__
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
AttributeError: type object 'object' has no attribute '__exit__'

It definitely isn't inheriting __exit__

Answer 1

The with statement opcode SETUP_WITH looks up __exit__ as a "special method lookup", which ignores __getattr__ and __getattribute__ on new-style classes (but not on old-style classes). See this mailing list thread for more information, where they discuss adding the special method lookup semantics to with (which they eventually do). See also special method lookup for new-style classes for a detailed discussion on why these special methods are looked up in this way.

In particular, special method lookup also bypasses __getattr__ on the type object. So, even though the documentation says the method is looked up as type(mgr).__exit__, this code doesn't work:

class M(type):
    def __getattr__(*args): return lambda: 0

class X(object):
    __metaclass__ = M

x = X()
type(x).__exit__ # works, returns a lambda

with x: pass # fails, AttributeError