AttributeError in "with" statement (3.2.2)

[88888 Dihedral]

On Wednesday, December 14, 2011 4:01:24 PM UTC+8, Steven D'Aprano wrote:
> On Wed, 14 Dec 2011 01:29:13 -0500, Terry Reedy wrote:
> 
> > 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.
> 
> I'm not sure that is correct... I don't think that there is anything "by 
> definition" about where methods live. Particularly not in Python where 
> instance methods can be attributes of the instance itself.
> 
> >>> class Test(object):
> ...     def method(self):
> ...             print("This method is an attribute of the class.")
> ... 
> >>> t = Test()
> >>> t.method()
> This method is an attribute of the class.
> >>>
> >>> import types
> >>> t.method = types.MethodType(
> ...     lambda self: print(
> ...     "This method is an attribute of the instance."), t)
> >>> t.method()
> This method is an attribute of the instance.
> 
> 
> So the normal lookup rules that apply to data attributes, namely 
> instance, then class, then superclasses, also applies to methods in 
> Python. In languages that don't allow that sort of thing, like Java, you 
> need to use convoluted design patterns like Dynamic Proxy to make it 
> work. In Python, you just create a method and attach it on the instance.
> 
> http://stackoverflow.com/questions/8260740/override-a-method-for-an-
> instance-of-a-class
> 
> But this doesn't apply for special dunder attributes like __exit__, for 
> speed reasons. (For new-style classes only, classic classes have no such 
> special casing. This makes automatic delegation a breeze in Python 2 with 
> classic classes, and a PITA in Python 3. Boo hiss.)
> 
> 
> 
> -- 
> Steven

In Python an instance of an object of a class can have its own method. 
A living object can use  those  methods in the class definition and 
can acquire a new method at runtime.