An environment variable is a named configuration value supplied by Linux to a running Python program. It keeps settings outside the code, so the same script can behave differently without editing its source. For a two-motor robot, separate values can compensate for motors that run at slightly different speeds and help the robot drive straighter. section 3.3 section 3.5
Use a clear shared prefix when naming variables. Names such as ROBO_DC_ADJUST_R and ROBO_DC_ADJUST_L are easy to find with `env | grep ROBO_`. Set a value in Bash with `export ROBO_DC_ADJUST_R="0.8"`, then start the Python script in that terminal session. section 3.5
Read configuration defensively in Python:
- Import `os`, which provides access to environment variables. section 3.5
- Treat every environment value as a string. Convert it to the type the program needs, such as `float` for a motor-power adjustment. section 3.5
- Use `os.environ.get(name, default)` for optional settings. Direct indexing with `os.environ[name]` raises `KeyError` when the name is absent. section 3.5
- Choose a safe default such as `1`, so an unset motor adjustment leaves the requested throttle unchanged. section 3.5
Adjust each motor without changing movement code
Read right and left adjustments once, store them by motor name, and apply the matching adjustment inside a reusable throttle function. `forward()` can still request equal throttle for both motors, while configuration scales each side independently. Set the right adjustment to `0.8` when that motor needs less power; leave an unset side at the default of `1`. section 3.5
Listing 3.2 envforward.py: Reading configuration values from environment variables
#!/usr/bin/env python3
from adafruit_crickit import crickit
import time
import os
DC_ADJUST_R = float(os.environ.get('ROBO_DC_ADJUST_R', '1'))
DC_ADJUST_L = float(os.environ.get('ROBO_DC_ADJUST_L', '1'))
ADJUST = dict(R=DC_ADJUST_R, L=DC_ADJUST_L)
MOTOR = dict(R=crickit.dc_motor_1, L=crickit.dc_motor_2)
def set_throttle(name, value):
MOTOR[name].throttle = value * ADJUST[name]
def forward():
set_throttle('R', 0.9)
set_throttle('L', 0.9)
time.sleep(0.2)
set_throttle('R', 0)
set_throttle('L', 0)
forward()
This runnable script reads optional motor-adjustment variables, converts them to floats, and applies them during forward motion. section 3.5
Environment variables can also select application behavior. A web application can set its Tornado `debug` setting from whether `ROBO_DEBUG` is defined, allowing development debugging behavior to be controlled outside the code base while keeping one script for development and production use. section 6.3.1
