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I want to convert a quaternion to a matrix and back again. The result has a different orientation than the quaternion from the beginning.

To reproduce the problem, set a quaternion.

ob = bpy.context.object
bpy.context.object.rotation_quaternion = mathutils.Quaternion((0.7795193195343018, 0.045188724994659424, 0.15823090076446533, 0.048953697085380554))

Then convert the quaternion to a matrix and apply it back to the object.

ob.matrix_world = ob.rotation_quaternion.to_matrix().to_4x4()

The object not only changes its orientation, but its scale.

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Your quaternion is not normalized, this can happen during interpolation.

Blender will still only view the quaternion as an orientation, even if it is not pure. However, the to_matrix function will convert the quaternion as it is.

To circumvent this, simply normalize the quaternion before converting it to a matrix.

ob.matrix_world = ob.rotation_quaternion.normalized().to_matrix().to_4x4()
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