On 12/14/2011 1:05 AM, Eric Snow wrote:
On Tue, Dec 13, 2011 at 10:42 PM, Steve Howell<showel...@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
foo.goodbye, aliased as foo.__exit__, is a *function* attribute of the
foo *instance*.
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?
To complement what Eric says below: The with statement is looking for an
instance *method*, which by definition, is a function attribute of a
*class* (the class of the context manager) that takes an instance of the
class as its first parameter. Notice that goodbye does not take any
parameter. The first parameter is conventionally called 'self', but that
is not a requirement. That it be there, that it expect to be bound to an
instance, and that the function be bound to the class are requirements.
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.
--
Terry Jan Reedy
--
http://mail.python.org/mailman/listinfo/python-list
