Calling len() on the scipy.spatial.transform.rotation.Rotation object with one rotation should return 1

[MatusGasparik]

When calling the len() function on the scipy.spatial.transform.rotation.Rotation object with exactly one rotation, it currently raises a TypeError instead of returning 1.

(Link to source code)

Reproducing code example:

from scipy.spatial.transform import Rotation as R

r = R.from_quat([1, 0, 0, 0])
print(len(r))    # raises TypeError

Error message:

TypeError                                 Traceback (most recent call last)
<ipython-input-227-c5f861eb1f8a> in <module>
      4 try:
----> 5     print(len(r))
      6 except Exception as e:

rotation.pyx in scipy.spatial.transform.rotation.Rotation.__len__()

TypeError: Single rotation has no len().

Calling len() on an object with 2 rotations returns 2 as expected:

r = R.from_quat([[1, 0, 0, 0],
                 [1, 0, 0, 0]])
print(len(r))    # returns 2

Moreover, when creating a rotation object using the from_euler() class method, it correctly returns 1 for one rotation around a single coordinate axis, but raises the above error again for a rotation around more than one coordinate axes:

r = R.from_euler('x', [0.1])
print(len(r))    # returns 1

r = R.from_euler('xz', [0.1, 0.1])
print(len(r))    # raises TypeError

Scipy/Numpy/Python version information:

import sys, scipy, numpy; print(scipy.__version__, numpy.__version__, sys.version_info)
1.6.0 1.19.5 sys.version_info(major=3, minor=9, micro=1, releaselevel='final', serial=0)

[AtsushiSakai]

I think the answer to this issue is in #12398

[rgommers]

This still works this way and seems inconsistent. I don't immediately see how gh-12398 answers whether this is intended or not - looks like a bug to me.

[WarrenWeckesser]

Based on the comments in #16663 (#16663 (comment) in particular) I think this is intended behavior. r = Rotation([1, 0, 0, 0]) is like a scalar rotation, so the object has no length, and len(r) should raise a TypeError. r2 = Rotation([[1, 0, 0, 0], [0, 1, 0, 0]]) is like a sequence of two rotations, and len(r2) is 2.

If my interpretation is correct, we can close this issue.