On 2021-06-21 12:49 p.m., Chris Angelico wrote:
> On Tue, Jun 22, 2021 at 1:44 AM Soni L. <fakedme...@gmail.com> wrote:
> >
> >
> >
> > On 2021-06-21 12:26 p.m., Stephen J. Turnbull wrote:
> > > Soni L. writes:
> > >
> > > > The trick to extension methods is that they're only available when you
> > > > explicitly use them.
> > >
> > > What does "explicitly use them" mean? How does this help avoid the
> > > kinds of problems we know that monkey-patching causes?
> >
> > Monkey-patching:
> >
> > ```py mod1.py
> > import foo
> >
> > foo.Bar.monkeymethod = ...
> > ```
> >
> > ```py mod2.py
> > import foo
> >
> > foo.Bar.monkeymethod = ...
> > ```
> >
> > "Extension methods":
> >
> > ```py mod1.py
> > import foo
> >
> > def __getattr__(o, attr):
> > if isinstance(o, foo.Bar) and attr == "monkeymethod":
> > return ...
> > return getattr(o, attr)
> > ```
> >
> > ```py mod2.py
> > import foo
> >
> > def __getattr__(o, attr):
> > if isinstance(o, foo.Bar) and attr == "monkeymethod":
> > return ...
> > return getattr(o, attr)
> > ```
> >
> > Note how the former changes foo.Bar, whereas the latter only changes the
> > module's own __getattr__. You can't have conflicts with the latter.
> > (Also note that this "module's own __getattr__" doesn't provide
> > extension methods by itself, but can be used as a mechanism to implement
> > extension methods.)
>
> So what you're saying is that, in effect, every attribute lookup has
> to first ask the object itself, and then ask the module? Which module?
> The one that the code was compiled in? The one that is currently
> running? Both?
>
> And how is this better than just using a plain ordinary function? Not
> everything has to be a method.
Quite the opposite. You ask the local module (the one that the code was
compiled in), and the module decides whether/when to ask the object itself.
In other words, every
foo.bar
would be sugar for
__getattr__(foo, "bar")
(where __getattr__ defaults to builtins.getattr) instead of being sugar for
<builtins.getattr>(foo, "bar")
(where <> is used to indicate that it doesn't quite desugar that way -
otherwise you'd need to recursively desugar it to
builtins.getattr(builtins, "getattr") which uh, doesn't work.)
>
> ChrisA
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