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What do Alloy's error messages mean and what should I change?

Every error in the Alloy physics engine (pip install alloy-engine) carries e.message, which says what went wrong, and e.fix, which says what to change. This page runs four mistakes people make and shows the exact text of each. For the error classes and two more examples, see Errors and fixes.

import textwrap
import alloy

def explain(call):
    try:
        call()
    except alloy.AlloyAPIError as e:
        print(textwrap.fill(e.message, width=110))
        print(textwrap.fill(e.fix, width=110))

scene = alloy.Scene(dt=0.01)
scene.add_ground()
scene.add_box("block", size=(0.1, 0.1, 0.1), pos=(0, 0, 0.05))

add_cloth: 'dough_wheat' is a paste (it yields and flows), and add_cloth makes sheets that bend and stretch

A paste material cannot be a sheet (alloy.SceneError). Give the paste a solid shape instead.

explain(lambda: scene.add_cloth("rag", size=(0.2, 0.2), pos=(0, 0, 0.3), material="dough_wheat"))

add_cloth: 'dough_wheat' is a paste (it yields and flows), and add_cloth makes sheets that bend and stretch
give the paste a solid shape instead, e.g. add_box(name, size=(x, y, depth), material='dough_wheat'); or pick
a sheet material (alloy.materials())

name 'block' is already in the scene

Names are unique, because every state key is built from them.

explain(lambda: scene.add_box("block", size=(0.1, 0.1, 0.1)))

name 'block' is already in the scene
pick another name (taken: ground, block)

device 'cuda:0' asked for but no CUDA GPU is visible

You asked for a GPU on a machine without one. This text appears on a CPU-only machine only.

explain(lambda: alloy.build(scene, device="cuda:0"))

device 'cuda:0' asked for but no CUDA GPU is visible
pass device='cpu' (slower) or run on a machine with an NVIDIA GPU and driver

joint 'shoulder' has no actuator, so a position target cannot move it

A target needs an actuator on the joint (alloy.SimulationError). Add the line the fix names to the MJCF.

ARM = """<mujoco><compiler angle="radian"/><worldbody><body name="link1" pos="0 0 0.1">
  <joint name="shoulder" type="hinge" axis="0 1 0" range="-2 2" damping="2"/>
  <geom type="capsule" fromto="0 0 0 0.3 0 0" size="0.02" mass="1"/></body></worldbody></mujoco>"""
arm_scene = alloy.Scene(dt=0.01)
arm_scene.add_robot("arm", mjcf=ARM, pos=(0, 0, 0.5))
explain(lambda: alloy.build(arm_scene).set_targets({"shoulder": 0.5}))

joint 'shoulder' has no actuator, so a position target cannot move it
add <position joint="..." kp="..."/> for it to the robot's MJCF <actuator> block

Keep in mind

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