Upgrade 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.

What changed?

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. This has significantly lowered the amount of work needed to update/maintain RobotPy – previously it took several volunteers over a month or two to do updates each year, and now a single person can do much of the work required within a week or so.

See this github issue for a longer discussion about this.

Upgrading from 2019 or prior

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 RobotPy in the normal way:

py -3 -m pip install --upgrade robotpy

See Computer Installation for more details about installing RobotPy on your computer.

Windows-specific notes

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!!
        # -> this is calling the base class, so if this is a Command it
        #    calls Command.__init__, a subsystem would call Subsystem.__init__,
        #    and so on.

        # 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.

What happened to physics and tests?

Test support is still not available yet.

The simulation ‘physics’ support has been significantly overhauled to integrate with the WPILib HAL/Simulation support. As of pyfrc 2020.1.0, the physics support has been updated and should work with the integrated field widget that comes with WPILib.

All of the physics example projects have been updated, but here are some particularly useful demos:

Additionally, see the PyFRC API docs for more information.

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()