Make Body and ReferenceFrame analogous

[sidhu1012]

Body and ReferenceFrame are analogous in dynamics, and all the joint classes , jointsmethod and class body follow it, but rest of physics module doesn't.

Example -

>>> A = Body('A')
>>> N = ReferenceFrame('N')
>>> A.dcm(N) # Works
>>> A.dcm(A) # Works

>>> N.dcm('A') # Error but should work

So where ever ReferenceFrame works on being passed as arg, passing body to same arg should also work.

It can be done simply by -

if isinstance(frame, Body):
    frame = Body.frame

cc - @moorepants

[mohajain]

I would like to work on this.

It can be done simply by -

if isinstance(frame, Body):
    frame = Body.frame

Where do I have to add this line?

[sidhu1012]

Where do I have to add this line?

In physics module wherever we can pass ReferenceFrame, we should be able to pass Body.

[mohajain]

The physics module is really vast, I'm unable to find such functions. Please guide me through, I'm new to sympy.

[sidhu1012]

The one's that I remember are Physics.mechanics.functions module(Inertia and all) , physics.mechanics.vector.frame (orient functions) and many more.

All of the functions would be in mechanics and vector sub module.

[mohajain]

Thanks a lot!
Do I have to change these lines

if not isinstance(frame, ReferenceFrame):
        raise TypeError('Please enter a valid ReferenceFrame')

to something like this

if not isinstance(frame, ReferenceFrame) and not isinstance(frame, Body):
        raise TypeError('Please enter a valid ReferenceFrame')

as well?

[sidhu1012]

Only change required is to add seperate check if frame's a body before checking frame.

[moorepants]

It would be nice if duck typing worked, i.e. add all methods to body that mimic ReferenceFrame.

[sidhu1012]

It would be nice if duck typing worked, i.e. add all methods to body that mimic ReferenceFrame.

Yes that's the end goal, Body should have everything possible that Particle, Rigid body and Reference Frame has.

[yasphy]

Is this issue still open?

[noob8boi]

I would like to work on this issue. I am pretty new to open source development so I think this problem will be a great way to start

[tjstienstra]

The current solutions here seem to focus on checking on every usage whether something is not a Body. Wouldn't it be better to instead change the Body class such that it responds as a ReferenceFrame?
I am currently experimenting with something like:

def __getattr__(self, name):
    if hasattr(self.frame, name):
        return self.frame.__getattribute__(name)
    else:
        raise AttributeError

This is why I use __getattr__. This part will solve the direct calls on the Body, when an attribute is needed like body.x. But it will give error on the isinstance checks. I have not yet figured out how to nicely fake a type, also not sure if we should even want it.

[tjstienstra]

I may have a better solution, since the solution I just proposed, will probably be too tricky to get working nicely. It is also a bit more like the current proposed implementation with isinstance, however it would for example also work for RigidBody.
The solution I propose can have two forms:

• Add a method get_frame to the classes, which should function as a ReferenceFrame (i.e. ReferenceFrame, Body, RigidBody). And instead of the isinstance we use frame = frame.get_frame().
• Replace the _check_frame calls for _get_frame (shown below)

def _get_checked_frame(other):
    frame = getattr(other, 'frame', other)
    _check_frame(frame)
    return frame

The advantage of these solutions is that it will be more future proof and it will also return the responsibility a bit more to the classes themselves, instead of to the client.

[sujantkumarkv]

Hey there!
I request a response on the status of this issue.

@tjstienstra @sidhu1012 @mohajain @yasphy @Infinil

[tjstienstra]

I cannot seem to find a discussion I had about whether it should entirely stay composition or not. I thought that the conclusion of that discussion was that it would be better to just leave it as composition and let them not become entirely analogous. However I cannot entirely confirm this, as I cannot find where that discussion took place.

Another argument to maybe close this issue is that the Body class might get deprecated for several reasons (as mentioned in #24256). The main argument is that it is more reasonable to that Particle and RigidBody inherit from a BodyBase, instead that Body inherits them both. This is more sensible from a physics point of view, but it is also nicer in programming, due to possible problems caused by multiple inheritance. Besides that it is actually possible to support Particle in the joints framework, so there is no real need for Body.

[sujantkumarkv]

Hey!
As far as I read (and please correct me): BodyBase has been implemented but Body hasn't been deprecated according to pull #24234.

So, as per this issue, we started off with making Body and ReferenceFrame analogous but currently stand on leaving it as composition and because Body would be deprecated sooner or later.
I came via GSOC Ideas'23 from Bug burn prioritizing to solve & close left issues.

So, with all said above, doesn't it fit that this issue can be closed now or after deprecating Body? and that we don't need further work on this? Please let me know or assign me if further work is required to close this.
Thanks.

[sujantkumarkv]

Body and ReferenceFrame are analogous in dynamics, and all the joint classes , jointsmethod and class body follow it, but rest of physics module doesn't.

Example -

>>> A = Body('A')
>>> N = ReferenceFrame('N')
>>> A.dcm(N) # Works
>>> A.dcm(A) # Works

>>> N.dcm('A') # Error but should work

So where ever ReferenceFrame works on being passed as arg, passing body to same arg should also work.

It can be done simply by -

if isinstance(frame, Body):
    frame = Body.frame

cc - @moorepants

I would also like to mention regarding the original issue:
A.dcm(N) doesn't work & neither does N.dcm(A), A.dcm(A) returns :: Matrix([[1, 0, 0], [0, 1, 0], [0, 0, 1]])

[sujantkumarkv]

Hey @tjstienstra,

Based on this comment of yours, I would think that it's best to close this issue as to keep things in order and clean.
Thanks.

[shishir-11]

I think this issue needs to be closed since Body has been deprecated