On Tue, Dec 15, 2020 at 12:49:26PM +0300, Paul Sokolovsky wrote:
> > Are you asking for a semantic difference (the two statements do
> > something different) or an implementation difference (the two
> > statements do the same thing in slightly different ways)?
>
> I'm asking for semantic difference, it's even in the post title.
So far all you have talked about is implementation differences such as
whether intermediate results are put on a stack or not, and differences
in byte-code from one version of Python to another.
> But
> the semantic difference in not in "what two statements do", but in
> "what two statements mean".
Right, so why are you wasting time talking about what they *do*, i.e.
whether they put intermediate results on the stack, or in a register?
> Difference in "doing" is entailed by
> difference in "meaning". And there may be no difference in "doing",
> but still difference in "meaning", as the original "1+2+3" vs
> "1+(2+3)" example was intended to show.
In the case of ints, there is no difference in meaning. For integers,
addition is associative, and the order does not matter.
So here you *say* you are talking about semantics, but you are actually
talking about implementation. With integers, the semantics of all of
these are precisely the same:
1 + 2 + 3
(1 + 2) + 3
1 + (2 + 3)
3 + (2 + 1)
etc. The order in which you *do* the additions makes no difference to
the semantics.
> > Implementation differences are fairly boring (to me):
>
> Right. How implementation points are brought into the discussion is to
> show the issue. As mentioned above, the actual progression is the
> opposite: first there's semantic meaning, then it's implemented. So,
> what's the semantic meaning of "a.b()" that it gets compiled with
> LOAD_METHOD bytecode?
- Look up the name "a" in the local namespace;
- look up the attribute name "b" according to a's MRO, including
the use of the descriptor protocol, slots, etc;
- call whatever object gets returned.
[...]
> > For what it is worth, Python 1.5 generates the exact same byte code
> > for both expressions; so does Python 3.9. However the byte code for
> > 1.5 and for 3.9 are different.
>
> Right, and the question is what semantic (not implementational!) shift
> happened in 3.7 (that's the point when it started to be compiled
> differently).
Absolutely none.
There was a semantic shift, but it was back in Python 2.2 when new style
classes and the descriptor protocol were introduced.
> > However, the semantics of the two expressions are more or less
> > identical in all versions of Python, regardless of the byte-code used.
>
> That's what I'd like to put under scrutiny.
Okay, the major semantic differences include:
- in Python 1.5, attribute name look-ups call `__getattr__` if the
name is not found in the object's MRO;
- in Python 3.9, attribute name look-ups first call `__getattribute__`
before checking the MRO and `__getattr__`;
- the MRO is calculated differently between 1.5 and 3.9;
- in 3.9, the descriptor protocol may be invoked;
- descriptors and the descriptor protocol did not exist in 1.5;
- there are a few differences in the possible types of `obj.meth`
between the versions, e.g. Python 1.5 had both bound and unbound
instance methods, while Python 3.9 does not.
There may be other differences, but those are the most important
ones I can remember.
> > (I say "more or less" because there may be subtle differences between
> > versions, relating to the descriptor protocol, or lack there of, old
> > and new style classes, attribute lookup, and handling of unbound
> > methods.)
>
> Right, and exactly those "subtle differences" is what I'd like to
> discuss.
See above.
> The level of abstraction I'm talking about is where you look not just
> at "`(expression)` vs `expression`" but at:
>
> expression <op> expression vs expression <op> (expression)
That is an extremely straight-forward change in execution order. Whether
that makes any semantic difference depends on whether the operations
involved are associative or not.
> Where <op> is an arbitrary operator. As we already concluded, those do
> have difference, even considering such a simple operator as "+".
>
> So... what can we say about the difference between a.b() and (a.b)()
> then?
There isn't one.
Even though the `.` (dot) and `x()` (call) are not actual operators, we
can treat them as pseudo-operators. According to Python's precedence
rules, the first expression `a.b()` is the same as:
- lookup name a
- lookup attribute b
- call
and the second `(a.b)()` is:
- lookup name a
- lookup attribute b
- call
which is precisely the same. The parens make no semantic difference.
Paul, I think you have fooled yourself by comparing two *different*
situations. You compare a use of parens where they change the order of
operations:
a + b + c
a + (b + c)
but you should be looking at this:
(a + b) + c # parens are redundant and make no difference
That is exactly equivalent to your method example:
(obj.meth)() # left pair of parens are redundant
> > > it might
> > > be helpful to add the 3rd line:
> > >
> > > t = obj.meth; t()
> >
> > That clearly has different semantics from the first two: it has the
> > side-effect of binding a value to the name t.
>
> But that's yet another good argument to introduce block-level scoping
> to Python (in addition to already stated great arguments), because then,
>
> (a.b)()
>
> will be *exactly* equivalent to (inside a function):
>
> if 1:
> const t = a.b
> t()
You are changing the rules of the discussion as you go. You said nothing
about a hypothetical new feature of block scopes and constants. You gave
an example of existing Python code:
t = obj.meth; t()
There is no if block here, no new scope, no constants. t is a plain old
ordinary local variabe in the current scope.
> Hopefully, that was answered above. To end the post with the summary,
> I'm suggesting that there's difference between:
>
> expression <op> expression vs expression <op> (expression)
>
> Which is hopefully hard to disagree with.
There may or may not be a difference, depending on the associativity and
precedence rules involved.
> Then I'm asking, how consistent are we with understanding and
> appreciating that difference, taking the example of:
>
> a.b() vs (a.b)()
Where as this example has only a single interpretation of the
precedence:
- lookup a in the current scope;
- lookup attribute b on a;
- call the result.
There's no other order of operations available, so no way for the parens
to change the order of operations:
- you cannot call a.b until you have looked up b on a;
- you cannot lookup b on a until you have looked up a.
Let's be concrete:
s = "hello world"
s.upper() # returns "HELLO WORLD"
There is only one possible order of operations:
- lookup s;
- lookup "upper" on s;
- call the resulting method.
You cannot use parens to change the order of operations:
- call the resulting method first (what method?)
- lookup "upper" on s (what's s?)
- lastly lookup s (too late!)
--
Steve
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