AttributeError in "with" statement (3.2.2)

[Eric Snow]

On Tue, Dec 13, 2011 at 10:42 PM, Steve Howell <showell30 at yahoo.com> wrote:
> I'm using Python 3.2.2, and the following program gives me an error
> that I don't understand:
>
> class Foo:
>  pass
>
> foo = Foo()
> foo.name = "Steve"
>
> def add_goodbye_function(obj):
>  def goodbye():
>    print("goodbye " + obj.name)
>  obj.goodbye = goodbye
>
> add_goodbye_function(foo)
> foo.goodbye() # outputs goodbye Steve
> foo.__exit__ = foo.goodbye
> foo.__exit__() # outputs goodbye Steve
>
> with foo: # fails with AttributeError:  __exit__
>  print("doing stuff")
>
> I am dynamically adding an attribute __exit__ to the variable foo,
> which works fine when I call it directly, but it fails when I try to
> use foo as the expression in the with statement.  Here is the full
> output:
>
>> python3 with.coffee
> goodbye Steve
> goodbye Steve
> Traceback (most recent call last):
>  File "with.coffee", line 17, in <module>
>    with foo: # fails with AttributeError:
> AttributeError: __exit__
>
> What am I doing wrong?

That is a tricky one.

As with many of the special methods (start and end with __) in Python,
the underlying mechanism in the interpreter is directly pulling the
function from the class object.  It does not look to the instance
object for the function at any time.  See
http://docs.python.org/reference/datamodel.html#special-method-lookup-for-new-style-classes.

-eric