Steve Howell 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__
If you don't know it yet, contextlib.contextmanager offers a highlevel
alternative:
>>> @contextmanager
... def goodbye(obj):
... try:
... yield obj
... finally:
... print("goodbye", obj.name)
...
>>> with goodbye(Foo()) as foo:
... print("hello", foo.name)
...
hello Steve
goodbye Steve
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