2020 Notes

Here are things to know about 2020 that are different from prior years. If you find things that are different that aren’t in this list, please submit a bug report.

Why is everything so different this year?

From 2015-2020, RobotPy maintained a pure python version of WPILib and bindings around third party vendor libraries. However, maintaining RobotPy is a lot of work, and as WPILib and third party vendors add even more features it was becoming slowly impossible to keep up.

In 2020, we switched to using mostly automatically generated wrappers around C++ libraries. Ideally, once completed, this will significantly lower the amount of work needed to update/maintain RobotPy in the future. Unfortunately in the short term, there’s a lot of work needed to get there (however – we’re most of the way there as of this writing!).

See this github issue for a longer discussion about this.

2020 has been a bit bumpy, but with your help hopefully we can smooth out the rough spots and make 2021 a seamless transition!

Upgrading from prior years

Before installing 2020 software (or after if you forgot), you should uninstall the following packages manually:

py -3 -m pip uninstall robotpy-hal-sim robotpy-hal-base

After that, you can install pyfrc et al in the normal way:

py -3 -m pip install --upgrade pyfrc

See Installing pyfrc for more details about installing RobotPy on your computer.

Windows-specific notes

  • The Visual Studio 2019 redistributable package is required to be installed.
  • CTRE and REV do not support simulation in 32-bit programs, so you must have a 64-bit Python installed.

OSX-specific notes

CTRE and REV do not support simulation on OSX, so these packages do not work at this time. You can work around this by checking for an import error:

    import ctre
except ImportError:
    ctre = None


if ctre is not None:

A mock solution could be provided for simulation. If you’re interested in developing something, contact us!

Linux specific notes

Linux requires Ubuntu 18.04 or a distribution with an equivalent (or newer) glibc installation. See linux installation page for more information.

Command framework

As of 2020, the command frameworks are distributed as separate packages. See the install page for details.

Class inheritance

When inheriting from RobotPy provided objects, and you provide your own __init__ function, you must call the base class’ __init__ function. If you don’t have your own __init__ function, there is no need to add one, Python will do the right thing automatically.

Here are some examples using the command framework objects, but this applies to any RobotPy object that you might inherit from:

from wpilib.command import Command

class GoodCommand(Command):

    # no custom __init__, nothing extra required

    def foo(self):

class AlsoGoodCommand(Command):

    def __init__(self):

        # Call this first!!

        # custom stuff here
        self.my_cool_thing = 1

class BadCommand(Command):
    def __init__(self):
        self.my_cool_thing = 1

        # BAD!! you forgot to call Command.__init__, which will result
        # in a difficult to diagnose crash!

class MaybeBadCommand(Command):
    def __init__(self):
        # This is not recommended, as it may fail in some cases
        # of multiple inheritance. See below
        self.my_cool_thing = 1

The pybind11 documentation recommends against using super().__init__():

Note that a direct __init__ constructor should be called, and super() should not be used. For simple cases of linear inheritance, super() may work, but once you begin mixing Python and C++ multiple inheritance, things will fall apart due to differences between Python’s MRO and C++’s mechanisms.

Where is physics and tests?

We are working on the 2020 implementation, and then once completed will focus on adding support for these.

Some community members have expressed interest in fixing support for testing/physics, but the work still has not been completed.

My code segfaulted and there’s no Python stack trace!

We are still working through the bugs, and when you find something like this here’s what you can do:

First, figure out where the code is crashing. Traditional debugging techniques apply here, but a simple way is to just delete and/or comment out things until it no longer fails. Then add the last thing back in and verify that the code still crashes.

Advanced users can compile a custom version of the robotpy libraries with symbols and use gdb to get a full stack trace (documentation TBD).

Once you’ve identified where it crashes, file a bug on github and we can help you out.

Common causes

Python objects are reference counted, and sometimes when you pass one directly to a C++ function without retaining a reference a crash can occur:

class Foo:
    def do_something(self):

In this example, Thing is immediately destroyed after some_function returns (because there are no references to it), but some_function (or something else) tries to use the object after it is destroyed. This causes a segfault or memory access exception of some kind.

These are considered bugs in RobotPy code and if you report an issue on github we can fix it. However, as a workaround you can retain a reference to the thing that you created and that often resolves the issue:

class Foo:
    def do_something(self):
        self.thing = Thing()