z_compensations: 0.0, 0.0, 0.0 in axis_twist_compensation appears to read 0.0 value as None, and Kalico crashes with a TypeError

[synthead]

Related to #589

When using z_compensations: 0.0, 0.0, 0.0 in the [axis_twist_compensation] section, like so:

[axis_twist_compensation]                                                                                                                                                                                          
speed: 2000

# X-axis compensation                                                                                                                                                                                              
calibrate_start_x: 0                                                                                                                                                                                               
calibrate_end_x: 230                                                                                                                                                                                               
calibrate_y: 115
z_compensations: 0.0, 0.0, 0.0                                                                                                                                                                                     

# Y-axis compensation                                                                                                                                                                                              
calibrate_start_y: 43.3                                                                                                                                                                                            
calibrate_end_y: 230                                                                                                                                                                                               
calibrate_x: 115                                                                                                                                                                                                   
zy_compensations: 0.0, 0.0, 0.0                                                                                                                                                                                    

...Kalico starts, the bed and extruder will warm up, it'll home, but when Kalico attempts to perform a BED_MESH_CALIBRATE, this exception is raised (truncated)

  File "/usr/lib/kalico/klippy/extras/axis_twist_compensation.py", line 87, in _get_interpolated_z_compensation
    spacing = (comp_end - comp_start) / (sample_count - 1)
               ~~~~~~~~~^~~~~~~~~~~~
TypeError: unsupported operand type(s) for -: 'NoneType' and 'NoneType'

Here is the full output (with systemd log prefixes)

Feb 27 16:50:29 enderng python[5480]: Upcoming (66572): 'MESH_CALIBRATE ADAPTIVE=1\n; G1 Z30.0 F3000 ; Move Z Axis up little to
 prevent scratching of Heat Bed\nG1 X0.1 Y20 Z0.3 F5000.0 ; '
Feb 27 16:50:29 enderng python[5480]: INFO:root:gcode state: absolute_coord=True absolute_extrude=True base_position=[0.0, 0.0,
 0.0, 0.0] last_position=[9.47500000004721, 0.7000000000531648, 4.1662500000001845, 0.0] homing_position=[0.0, 0.0, 0.0, 0.0] s
peed_factor=0.016666666666666666 extrude_factor=1.0 speed=25.0
Feb 27 16:50:29 enderng python[5480]: INFO:root:Reactor garbage collection: (8012.713696271, 0.0, 0.0)
Feb 27 16:50:29 enderng python[5480]: WARNING:root:Internal error on command:"BED_MESH_CALIBRATE"
Feb 27 16:50:29 enderng python[5480]: ERROR:root:virtual_sdcard dispatch
Feb 27 16:50:29 enderng python[5480]: Traceback (most recent call last):
Feb 27 16:50:29 enderng python[5480]:   File "/usr/lib/kalico/klippy/extras/virtual_sdcard.py", line 322, in work_handler
Feb 27 16:50:29 enderng python[5480]:     self.gcode.run_script(line)
Feb 27 16:50:29 enderng python[5480]:     ~~~~~~~~~~~~~~~~~~~~~^^^^^^
Feb 27 16:50:29 enderng python[5480]:   File "/usr/lib/kalico/klippy/gcode.py", line 333, in run_script
Feb 27 16:50:29 enderng python[5480]:     self._process_commands(script.split("\n"), need_ack=False)
Feb 27 16:50:29 enderng python[5480]:     ~~~~~~~~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
Feb 27 16:50:29 enderng python[5480]:   File "/usr/lib/kalico/klippy/gcode.py", line 310, in _process_commands
Feb 27 16:50:29 enderng python[5480]:     handler(gcmd)
Feb 27 16:50:29 enderng python[5480]:     ~~~~~~~^^^^^^
Feb 27 16:50:29 enderng python[5480]:   File "/usr/lib/kalico/klippy/gcode.py", line 221, in func
Feb 27 16:50:29 enderng python[5480]:     return origfunc(self._get_extended_params(params))
Feb 27 16:50:29 enderng python[5480]:   File "/usr/lib/kalico/klippy/extras/bed_mesh.py", line 896, in cmd_BED_MESH_CALIBRATE
Feb 27 16:50:29 enderng python[5480]:     self.probe_helper.start_probe(gcmd)
Feb 27 16:50:29 enderng python[5480]:     ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^^^^^^
Feb 27 16:50:29 enderng python[5480]:   File "/usr/lib/kalico/klippy/extras/probe.py", line 617, in start_probe
Feb 27 16:50:29 enderng python[5480]:     pos = probe.run_probe(gcmd)
Feb 27 16:50:29 enderng python[5480]:   File "/usr/lib/kalico/klippy/extras/probe.py", line 229, in run_probe
Feb 27 16:50:29 enderng python[5480]:     pos = self._probe(speed)
Feb 27 16:50:29 enderng python[5480]:   File "/usr/lib/kalico/klippy/extras/probe.py", line 173, in _probe
Feb 27 16:50:29 enderng python[5480]:     z_compensation = axis_twist_compensation.get_z_compensation_value(
Feb 27 16:50:29 enderng python[5480]:         pos
Feb 27 16:50:29 enderng python[5480]:     )
Feb 27 16:50:29 enderng python[5480]:   File "/usr/lib/kalico/klippy/extras/axis_twist_compensation.py", line 63, in get_z_comp
ensation_value
Feb 27 16:50:29 enderng python[5480]:     self._get_interpolated_z_compensation(
Feb 27 16:50:29 enderng python[5480]:     ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^
Feb 27 16:50:29 enderng python[5480]:         pos[0],
Feb 27 16:50:29 enderng python[5480]:         ^^^^^^^
Feb 27 16:50:29 enderng python[5480]:     ...<2 lines>...
Feb 27 16:50:29 enderng python[5480]:         self.compensation_end_x,
Feb 27 16:50:29 enderng python[5480]:         ^^^^^^^^^^^^^^^^^^^^^^^^
Feb 27 16:50:29 enderng python[5480]:     )
Feb 27 16:50:29 enderng python[5480]:     ^
Feb 27 16:50:29 enderng python[5480]:   File "/usr/lib/kalico/klippy/extras/axis_twist_compensation.py", line 87, in _get_inter
polated_z_compensation
Feb 27 16:50:29 enderng python[5480]:     spacing = (comp_end - comp_start) / (sample_count - 1)
Feb 27 16:50:29 enderng python[5480]:                ~~~~~~~~~^~~~~~~~~~~~
Feb 27 16:50:29 enderng python[5480]: TypeError: unsupported operand type(s) for -: 'NoneType' and 'NoneType'
Feb 27 16:50:29 enderng python[5480]: INFO:root:MCU 'usb-adxl345' shutdown: Command request

0.0 should be a valid value. It appears that a 0.0 value is read as None here.

[synthead]

Okay, this is pretty confusing, but I got to the bottom of it. This is where the compensation values are read:

kalico/klippy/extras/axis_twist_compensation.py

Lines 30 to 38 in 0104218

	self.z_compensations = config.getlists(
	"z_compensations", default=[], parser=float
	)
	self.compensation_start_x = config.getfloat(
	"compensation_start_x", default=None
	)
	self.compensation_end_x = config.getfloat(
	"compensation_end_x", default=None
	)

When the values are being read from the config, AxisTwistCompensation expects:

• compensation_start_x
• compensation_end_x

...but when a calibration starts, these values are read from the config from AxisTwistCompensation instead:

• calibrate_start_x
• calibrate_end_x

This makes sense, because you can adjust calibrate_start_x and leave the old compensation in-place. This will ensure that nothing changes until BED_MESH_CALIBRATE is called again, and a new twist calibration is written.

This should be documented when fixing this issue:

• z_compensations and zy_compensations values in axis_twist_compensation section is undocumented #589