On Dec 14, 12:01 am, Steven D'Aprano <steve
+comp.lang.pyt...@pearwood.info> wrote:
> [...]
>
> 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.)
Thanks to all who responded. Basically, I think the special casing of
the "dunder" attributes threw me off.
Typically, I would just build context managers from a class, but I
wanted to experiment with instances that were built up without the
standard Python class mechanisms, instead following a Javascript-like
closure-based object creation model.
This is what I came up with:
class WithWrapper:
def __init__(self, obj):
self.obj = obj
def __enter__(self):
self.obj['enter']()
def __exit__(self, *args):
self.obj['exit'](*args)
def greeter_context(hello, goodbye):
return {
'enter': lambda: print("---\n" + hello),
'exit': lambda *args: print(goodbye)
}
gc_french = greeter_context("salut", "a plus tard")
with WithWrapper(gc_french):
print("doing stuff")
gc_slang = greeter_context("yo", "later")
with WithWrapper(gc_slang):
print("doing stuff")
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