Skip to content

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:

 1
 2
 3
 4
 5
 6
 7
 8
 9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
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

Move farther (four feet) or make a larger square

Move at a slower speed (how slow can you go and still move in a straight line?)