AttributeError in "with" statement (3.2.2)

[88888 Dihedral]

On Thursday, December 15, 2011 12:08:32 AM UTC+8, 88888 Dihedral wrote:
> 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.

Therefore, it is possible for an object to build its decision tree of 
actions toward a problem of various parameters in the run time.