<html>
<head>
<meta http-equiv="Content-Type" content="text/html; charset=utf-8">
</head>
<body text="#000000" bgcolor="#FFFFFF">
<p><br>
</p>
<br>
<div class="moz-cite-prefix">On 2017-10-29 02:28 AM, Nick Coghlan
wrote:<br>
</div>
<blockquote type="cite"
cite="mid:CADiSq7eNnMG+oFeVmxUKZTaKRcvcLd4u8rrb-meYKWG+2X6Bpw@mail.gmail.com">
<div dir="ltr">
<div class="gmail_extra">
<div class="gmail_quote">On 29 October 2017 at 12:25, Brendan
Barnwell <span dir="ltr"><<a
href="mailto:brenbarn@brenbarn.net" target="_blank"
moz-do-not-send="true">brenbarn@brenbarn.net</a>></span>
wrote:<br>
<blockquote class="gmail_quote" style="margin:0px 0px 0px
0.8ex;border-left:1px solid
rgb(204,204,204);padding-left:1ex"><span class="gmail-">On
2017-10-28 19:13, Soni L. wrote:<br>
<blockquote class="gmail_quote" style="margin:0px 0px
0px 0.8ex;border-left:1px solid
rgb(204,204,204);padding-left:1ex">
And to have all cars have engines, you'd do:<br>
<br>
class Car:<br>
def __init__(self, ???):<br>
self[Engine] = GasEngine()<br>
<br>
car = Car()<br>
car[Engine].kickstart() # kickstart gets the car as
second argument.<br>
<br>
And if you can't do that, then you can't yet do what
I'm proposing, and<br>
thus the proposal makes sense, even if it still needs
some refining...<br>
</blockquote>
<br>
</span>
As near as I can tell you can indeed do that,
although it's still not clear to me why you'd want to.
You can give Car a __getitem__ that on-the-fly generates
an Engine object that knows which Car it is attached to,
and then you can make Engine.kickstart a descriptor that
knows which Engine it is attached to, and from that can
figure out which Car it is attached to.<br>
</blockquote>
<div><br>
</div>
Right, I think a few different things are getting confused
here related to how different folks use composition.</div>
<div class="gmail_quote"><br>
</div>
<div class="gmail_quote">For most data modeling use cases, the
composition model you want is either a tree or an acyclic
graph, where the subcomponents don't know anything about the
whole that they're a part of. This gives you good component
isolation, and avoids circular dependencies.</div>
<div class="gmail_quote"><br>
</div>
<div class="gmail_quote">However, for other cases, you *do*
want the child object to be aware of the parent - XML etrees
are a classic example of this, where we want to allow
navigation back up the tree, so each node gains a reference
to its parent node. This often takes the form of a
combination of delegation (parent->child references) and
dependency inversion (child->parent reference).</div>
<div class="gmail_quote"><br>
</div>
<div class="gmail_quote">For the car/engine example, this
relates to explicitly modeling the relationship whereby a
car can have one or more engines (but the engine may not
currently be installed), while an engine can be installed in
at most one car at any given point in time.<br>
</div>
<div class="gmail_quote"><br>
</div>
<div class="gmail_quote">You don't even need the descriptor
protocol for that though, you just need the subcomponent to
accept the parent reference as a constructor parameter:<br>
</div>
<div class="gmail_quote"><br>
</div>
<div class="gmail_quote">
<div class="gmail_quote"> class Car:<br>
def __init__(self, engine_type):<br>
self.engine = engine_type(self)</div>
<br>
<div class="gmail_quote">However, this form of explicit
dependency inversion wouldn't work as well if you want to
be able to explicitly create an "uninstalled engine"
instance, and then pass the engine in as a parameter to
the class constructor:</div>
<div class="gmail_quote"><br>
</div>
<div class="gmail_quote">
<div class="gmail_quote"> class Car:<br>
def __init__(self, engine):<br>
self.engine = engine # How would we ensure the
engine is marked as installed here?<br>
</div>
</div>
</div>
<div class="gmail_quote"><br>
</div>
<div class="gmail_quote">As it turns out, Python doesn't need
new syntax for this either, as it's all already baked into
the regular attribute access syntax, whereby descriptor
methods get passed a reference not only to the descriptor,
but *also* to the object being accessed: <a
href="https://docs.python.org/3/howto/descriptor.html#descriptor-protocol"
moz-do-not-send="true">https://docs.python.org/3/howto/descriptor.html#descriptor-protocol</a></div>
<div class="gmail_quote"><br>
</div>
<div class="gmail_quote">And then the property builtin lets
you ignore the existence of the descriptor object entirely,
and only care about the original object, allowing the above
example to be written as:</div>
<div class="gmail_quote"><br>
</div>
<div class="gmail_quote">
<div class="gmail_quote">
<div class="gmail_quote"> class Car:<br>
def __init__(self, engine):<br>
self.engine = engine # This implicitly marks the
engine as installed<br>
</div>
</div>
<div class="gmail_quote"><br>
</div>
</div>
@property</div>
<div class="gmail_extra"> def engine(self):</div>
<div class="gmail_extra"> return self._engine</div>
<div class="gmail_extra"><br>
</div>
<div class="gmail_extra"> @engine.setter</div>
<div class="gmail_extra"> def engine(self, engine):</div>
<div class="gmail_extra"> if engine is not None:<br>
</div>
<div class="gmail_extra"> if self._engine is not
None:</div>
<div class="gmail_extra"> raise
RuntimeError("Car already has an engine installed")<br>
</div>
<div class="gmail_extra">
<div class="gmail_extra"> if engine._car is not
None:</div>
<div class="gmail_extra"> raise
RuntimeError("Engine is already installed in another car")</div>
<div class="gmail_extra">
<div class="gmail_extra"> engine._car = self<br>
self._engine = engine<br>
</div>
</div>
</div>
<div class="gmail_extra">
<div class="gmail_quote"><br>
</div>
<div class="gmail_quote">
car = Car(GasEngine())</div>
<div class="gmail_quote"><br>
</div>
<div class="gmail_quote">ORMs use this kind of descriptor
based composition management extensively in order to
reliably model database foreign key relationships in a way
that's mostly transparent to users of the ORM classes.<br>
</div>
</div>
</div>
</blockquote>
<br>
And this is how you miss the whole point of being able to
dynamically add/remove arbitrary components on objects you didn't
create, at runtime.<br>
<br>
Someone gave me this code and told me it explains what I'm trying to
do: <a class="moz-txt-link-freetext" href="https://repl.it/NYCF/3">https://repl.it/NYCF/3</a><br>
<br>
class T:<br>
pass<br>
<br>
class C:<br>
pass<br>
<br>
c = C()<br>
<br>
#c.[T] = 1<br>
c.__dict__[T] = 1<br>
<br>
I'd also like to add:<br>
<br>
def someone_elses_lib_function(arbitrary_object):<br>
#arbitrary_object.[T] = object()<br>
arbitrary_object.__dict__[T] = object()<br>
<br>
and<br>
<br>
def another_ones_lib_function(arbitrary_object):<br>
#if arbitrary_object.[T]:<br>
if arbitrary_object.__dict__[T]:<br>
#arbitrary_object.[T].thing()<br>
arbitrary_object.__dict__[T].thing(arbitrary_object)<br>
<br>
<blockquote type="cite"
cite="mid:CADiSq7eNnMG+oFeVmxUKZTaKRcvcLd4u8rrb-meYKWG+2X6Bpw@mail.gmail.com">
<div dir="ltr">
<div class="gmail_extra"><br>
<div class="gmail_quote">Cheers,</div>
<div class="gmail_quote">Nick.<br>
</div>
<br>
-- <br>
<div class="gmail_signature">Nick Coghlan | <a
href="mailto:ncoghlan@gmail.com" target="_blank"
moz-do-not-send="true">ncoghlan@gmail.com</a> |
Brisbane, Australia</div>
</div>
</div>
<br>
<fieldset class="mimeAttachmentHeader"></fieldset>
<br>
<pre wrap="">_______________________________________________
Python-ideas mailing list
<a class="moz-txt-link-abbreviated" href="mailto:Python-ideas@python.org">Python-ideas@python.org</a>
<a class="moz-txt-link-freetext" href="https://mail.python.org/mailman/listinfo/python-ideas">https://mail.python.org/mailman/listinfo/python-ideas</a>
Code of Conduct: <a class="moz-txt-link-freetext" href="http://python.org/psf/codeofconduct/">http://python.org/psf/codeofconduct/</a>
</pre>
</blockquote>
<br>
</body>
</html>