PyDy example fails

[Yuri Matiyasevich]

  I cannot reproduce program from

https://pydy.readthedocs.io/en/stable/

(Section Usage, Derive the system)

I got:

Traceback (most recent call last):
  File "/home/yumat/MODULES/PY_DY/pend.py", line 47, in <module>
    fr, frstar = kane.kanes_equations(forces, particles)     # Generate EoM's fr + frstar = 0
  File "/home/yumat/PYTHON_ENVIRONMENT_1/lib/python3.7/site-packages/sympy/physics/mechanics/kane.py", line 536, in kanes_equations
    fr = self._form_fr(loads)
  File "/home/yumat/PYTHON_ENVIRONMENT_1/lib/python3.7/site-packages/sympy/physics/mechanics/kane.py", line 282, in _form_fr
    vel_list, f_list = _f_list_parser(fl, N)
  File "/home/yumat/PYTHON_ENVIRONMENT_1/lib/python3.7/site-packages/sympy/physics/mechanics/functions.py", line 724, in _f_list_parser
    vel_list, f_list = unzip(list(flist_iter()))
  File "/home/yumat/PYTHON_ENVIRONMENT_1/lib/python3.7/site-packages/sympy/physics/mechanics/functions.py", line 711, in flist_iter
    obj, force = pair
TypeError: cannot unpack non-iterable Particle object

This occurs on two different machines, and similar errors happened with other examples.

What could be wrong?

Yuri Matiyasevich

[Andy Froncioni]

I get the same error.   :-(

[Jason Moore]

kanes_equations() had a deprecation some years back. The API is now:

fr, frstar = kane.kanes_equations(particles, forces)

Here is the documentation in SymPy: https://docs.sympy.org/latest/modules/physics/mechanics/api/kane_lagrange.html#sympy.physics.mechanics.kane.KanesMethod.kanes_equations

And more info on the deprecation:

https://github.com/sympy/sympy/issues/10945

Jason

moorepants.info
+01 530-601-9791

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[Andy Froncioni]

Thanks a bunch, Jason!  Hope you're well, buddy.

[Jason Moore]

Andy,

Yes, doing good. I started a project with Garmin/Tacx so maybe we'll connect sometime in the future.

Also, I just wrote this book for my class that uses SymPy mechanics:

https://moorepants.github.io/learn-multibody-dynamics/

It may also be a good resource.

Jason

moorepants.info
+01 530-601-9791

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