Organizing code into files

Your program is getting longer, so let’s make it easier to find things. We’ll put the intake in one file and the program that uses it in another. A Python file you import is called a module. The behavior stays the same; we’re giving the code a clearer place to live.

Splitting a program into modules¶

Create intake.py in your assignment folder and move your Intake class there. In a new robot_model.py, write from intake import Intake. This means “from the module named intake, make the name Intake available here.” The import uses the module name without .py. The spelling and capitalization must match.

An import also executes top-level statements in that module on its first import, so don’t leave a surprise demonstration running underneath your reusable class. Keep the class in intake.py and the demonstration in robot_model.py.

Composition means building an object out of other objects. A robot has an intake, so a Robot class can hold an Intake instance. The robot decides when to request an action; the intake owns the details of its own behavior. This is the same relationship we’ll use with a subsystem later.

Read it before running it¶

Put this worked example in student-work/foundations/modules_and_composition.py. Read it once and write down what you think it prints. Then save and run python student-work/foundations/modules_and_composition.py using your environment’s Python.

# This small standalone example demonstrates composition without needing another file.
class Intake:
    def __init__(self):
        self.output = 0.0

    def stop(self):
        self.output = 0.0

class Robot:
    def __init__(self):
        self.intake = Intake()

robot = Robot()
robot.intake.output = 0.4
robot.intake.stop()
print("Robot intake:", robot.intake.output)

Read robot.intake.stop() from left to right: find the robot object, get its intake attribute, call that intake’s stop method. It prints zero. When you split this example into files, the relationship doesn’t change; the import simply makes the class definition available where you need it.

Run your file from the repository root with python student-work/foundations/robot_model.py. Python can find a sibling intake.py beside the script. Don’t name your file math.py, wpilib.py, or commands2.py: it can hide a library with the same name.

Try it yourself¶

Before moving on¶

Explain what happens during import and which object owns output. Show that changing the intake logic in one file changes the demonstration using it.