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Using Variables

Variable Naming Rules

  • Must start with a letter or an underscore
  • Can only contain alpha-numeric characters and underscores (A-z, 0-9, and _)

  • Case sensitive (Delay is not the same as delay)

    Challenge

    Which Variable Names Are Legal?

    1. MrRoberts
    2. _goodbye!
    3. 2For1
    4. Mr.Tannenbaum
    5. X
    6. _wait
    7. _2

Python is both a Strongly Type and Dynamically Type Language

  • Strong typing means that the type of a value doesn't change in unexpected ways. A string containing only digits doesn't magically become a number. Every change of type requires an explicit conversion.
  • Dynamic typing means that runtime objects (values) have a type, as opposed to static typing where variables have a type.

What Does That Mean?

  • Text (string): wait = "two"
  • Number with a decimal point (float): wait = 2.0
  • Number without a decimal point (int): wait = 2
  • A Collection (list): wait = ["Two", 2.0, 2]

For example, the duty_u16 method from the PWM library only accepts a positive integer.

So?

We can use variables to make driving the robot car simpler. For example, we can create the following variables (use upper case to denote constants):

  • SPEED = 32768
  • DELAY = 2
  • FREQUENCY = 1000

Now we can have the robot car easily move faster or further without changing the numbers in multiple places. The simple code we had last time becomes:

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from machine import Pin, PWM
from time import sleep

DELAY = 2
SPEED = 32768
FREQUENCY = 1000

# Motor definitions
right_forward = PWM(Pin(11))
right_forward.freq(FREQUENCY)
right_reverse = PWM(Pin(10))
right_reverse.freq(FREQUENCY)
left_forward = PWM(Pin(9))
left_forward.freq(FREQUENCY)
left_reverse = PWM(Pin(8))
left_reverse.freq(FREQUENCY)

# Wait before moving
sleep(DELAY)

# Move Forward
right_reverse.duty_u16(0)
left_reverse.duty_u16(0)
right_forward.duty_u16(SPEED)
left_forward.duty_u16(SPEED)
sleep(DELAY)

# Move Backwards
right_forward.duty_u16(0)
left_forward.duty_u16(0)
right_reverse.duty_u16(SPEED)
left_reverse.duty_u16(SPEED)
sleep(DELAY)

# Stop Moving
right_forward.duty_u16(0)
right_reverse.duty_u16(0)
left_forward.duty_u16(0)
left_reverse.duty_u16(0)

If we want to move further or change the speed we can simply change the value of the variables.

Challenge

Rewrite the main.py code to use variables

Make a larger square

Move at a different speed