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Best Practices

This section has a selection of Python-specific things that other teams have found to be good things to keep in mind to build robot code that works consistently, and to eliminate possible failures.

If you have things to add to this section, feel free to submit a pull request!

Make sure you're running the latest version of RobotPy!

Seriously. We try to fix bugs as we find them, and if you haven't updated recently, check to see if you're out of date! This is particularly true during build season.

Don't use the print statement/logger excessively

Printing output can easily take up a large proportion of your robot code CPU usage if you do it often enough. Try to limit the amount of things that you print, and your robot will perform better.

Instead, you may want to use this pattern to only print once every half second (or whatever arbitrary period):

# Put this in robotInit
self.printTimer = wpilib.Timer()
self.printTimer.start()

..

# Put this where you want to print
if self.printTimer.hasPeriodPassed(0.5):
    self.logger.info("Something happened")

Remember, during a competition you can't actually see the output of Netconsole (it gets blocked by the field network), so there's not much point in using these except for diagnostics off the field. In a competition, disable it.

Don't die during the competition!

If you've done any amount of programming in python, you'll notice that it's really easy to crash your robot code -- all you need to do is mistype something and BOOM you're done. When python encounters errors (or components such as WPILib or HAL), then what happens is an exception is raised.

Note

If you don't know what exceptions are and how to deal with them, you should read this

There's a lot of things that can cause your program to crash, and generally the best way to make sure that it doesn't crash is test your code. RobotPy provides some great tools to allow you to simulate your code, and to write unit tests that make sure your code actually works. Whenever you deploy your code using pyfrc, it tries to run your robot code's tests -- and this is to try and prevent you from uploading code that will fail on the robot.

However, invariably even with all of the testing you do, something will go wrong during that really critical match, and your code will crash. No fun. Luckily, there's a good technique you can use to help prevent that!

What you need to do is set up a generic exception handler that will catch exceptions, and then if you detect that the FMS is attached (which is only true when you're in an actual match), just continue on instead of crashing the code.

Note

Most of the time when you write code, you never want to create generic exception handlers, but you should try to catch specific exceptions. However, this is a special case and we actually do want to catch all exceptions.

Here's what I mean:

try:
    # some code goes here
except:
    if not self.isFmsAttached():
        raise

What this does is run some code, and if an exception occurs in that code block, and the FMS is connected, then execution just continues and hopefully everything continues working. However (and this is important), if the FMS is not attached (like in a practice match), then the raise keyword tells python to raise the exception anyways, which will most likely crash your robot. But this is good in practice mode -- if your driver station is attached, the error and a stack trace should show up in the driver station log, so you can debug the problem.

Now, a naive implementation would just put all of your code inside of a single exception handler -- but that's a bad idea. What we're trying to do is make sure that failures in a single part of your robot don't cause the rest of your robot code to not function. What we generally try to do is put each logical piece of code in the main robot loop (teleopPeriodic) in its own exception handler, so that failures are localized to specific subsystems of the robot.

With these thoughts in mind, here's an example of what I mean:

def teleopPeriodic(self):

    try:
        if self.joystick.getTrigger():
            self.arm.raise_arm()
    except:
        if not self.isFmsAttached():
            raise

    try:
        if self.joystick.getRawButton(2):
            self.ball_intake.()
    except:
        if not self.isFmsAttached():
            raise

    # and so on...

    try:
        self.robot_drive.arcadeDrive(self.joystick)
    except:
        if not self.isFmsAttached():
            raise

Note

In particular, I always recommend making sure that the call to your robot's drive function is in it's own exception handler, so even if everything else in the robot dies, at least you can still drive around.