RobotPy Sim -- Windows fatal exception: access violation

Technical Programming
#1

We are running our code in 2020 sim and receiving fatal exception error. The sim dies when it runs a command group and says:

Windows fatal exception: access violation

We are speculating it could be due to a change in the 2020 command package. We used requires(Subsystem) and are wondering if this could be the cause of it failing.

Any suggestions would be greatly appreciated!

#2

Could you share your code?

See also https://robotpy.readthedocs.io/en/latest/2020_notes.html - something will probably be relevant there.

#3

The error in the console of the robot when trying to upload the code is
File ‘path to the file on the rio/commands/do_winch.py’, line 13 in init

Line 13 in do_winch reads
self.requires(robot.winch)

I have a feeling the way to use self.requires has changed since the super().init() is not used for 2020 wpilib commands but I don’t know how to use it correctly anymore

#4

also we have checked the 2020 release notes and haven’t found anything, but maybe there’s something we missed

#5

Yeah, you’re missing __init__ calls to the superclass for all your Subsystems and Commands.

#6

Right, but here’s an example from the 2020 update notes

from wpilib.command import Command

class GoodCommand(Command):

# no custom __init__, nothing extra required

def foo(self):
    pass
#7

Yes, that example does not define its own __init__, so the superclass __init__ does get called.

#8

where does it get called in that example? Or should I just add this
Command.init(self)

?

#9

If you override __init__ (i.e. you define it in your subclass), then you need to call Command.__init__(self) etc. in your __init__.

2 Likes
#10

Okay I will try that thanks

#11

We tried to put Command.init(self) in commands and tried to put super().init_() in subsystems which didn’t work in the sim. So, we removed superinit from subsystems but this still didn’t run in the sim.

#12

We’re getting an error that says
init(): incompatible constructor arguments

#13

The rule of thumb is that you call the base class constructor directly. So if you have:

class Foo(Bar):
    pass

Then calling it’s constructor in your custom __init__ is going to be

class Foo(Bar):
    def __init__(self):
        Bar.__init__(self)

So a subsystem would call Subsystem.__init__, etc.

1 Like
#14

Also, I fixed some issues in your repo, and made a pull request. https://github.com/beavertronics/2020code5970/pull/1

1 Like
#15

Where can we find the arguments for the command group init? Also thanks for the pull

#16

This year’s command API documentation is at https://robotpy.readthedocs.io/projects/commands-v1/en/latest/api.html

#17

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