Learning Python 3 with the Linkbot/Using Modules

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Calendar Module

Here's this chapter's typing exercise (name it cal.py (import actually looks for a file named calendar.py and reads it in. If the file is named calendar.py and it sees a "import calendar" it tries to read in itself which works poorly at best.)):

import calendar
year = int(input("Type in the year number: "))
calendar.prcal(year)

And here is part of the output I got:

Type in the year number: 2001

                                 2001                                  

       January                  February                    March      

Mo Tu We Th Fr Sa Su      Mo Tu We Th Fr Sa Su      Mo Tu We Th Fr Sa Su
1  2  3  4  5  6  7                1  2  3  4                1  2  3  4     
8  9 10 11 12 13 14       5  6  7  8  9 10 11       5  6  7  8  9 10 11
15 16 17 18 19 20 21      12 13 14 15 16 17 18      12 13 14 15 16 17 18     
22 23 24 25 26 27 28      19 20 21 22 23 24 25      19 20 21 22 23 24 25     
29 30 31                  26 27 28                  26 27 28 29 30 31        

(I skipped some of the output, but I think you get the idea.) So what does the program do? The first line import calendar uses a new command import. The command import loads a module (in this case the calendar module). To see the commands available in the standard modules either look in the library reference for python (if you downloaded it) or go to http://docs.python.org/library/. If you look at the documentation for the calendar module, it lists a function called prcal that prints a calendar for a year. The line calendar.prcal(year) uses this function. In summary to use a module import it and then use module_name.function<code> for functions in the module. Another way to write the program is:

from calendar import prcal
 
year = int(input("Type in the year number: "))
prcal(year)

This version imports a specific function from a module. Here is another program that uses the Python Library (name it something like clock.py) (press Ctrl and the 'c' key at the same time to terminate the program):

from time import time, ctime
 
prev_time = ""
while True:
    the_time = ctime(time())
    if prev_time != the_time:
        print("The time is:", ctime(time()))
        prev_time = the_time

With some output being:

The time is: Sun Aug 20 13:40:04 2000
The time is: Sun Aug 20 13:40:05 2000
The time is: Sun Aug 20 13:40:06 2000
The time is: Sun Aug 20 13:40:07 2000

Traceback (innermost last):
 File "clock.py", line 5, in ?
    the_time = ctime(time())

KeyboardInterrupt

The output is infinite of course so I canceled it (or the output at least continues until Ctrl+C is pressed). The program just does a infinite loop (<code>True is always true, so while True: goes forever) and each time checks to see if the time has changed and prints it if it has. Notice how multiple names after the import statement are used in the line from time import time, ctime.

The Python Library contains many useful functions. These functions give your programs more abilities and many of them can simplify programming in Python.

Linkbot Buttons and the random module

The following program uses the random module to generate random numbers. Basic usage is like this:

import random # Imports the module
x = random.randint(0, 100)

The previous little code snipped produces a random number between 0 and 100 and puts it in the variable x. The arguments to randint() control the range of the random number generated. For instance, if you instead wanted a random number between -100 and +100, you could do

x = random.randint(-100, 100)

The following Linkbot program uses the random module to change the LED color to some random colors.

# File: 01_rand_color_button.py
 
# This program changes the Robots LED color every time you press the "B" button 
# on the robot.
 
import linkbot  #imports the module with the Linkbot commands
import random   #imports a random integer generator
 
robotID = input('Enter robot ID: ') #prompts user to enter Linkbot ID
robot = linkbot.CLinkbot(robotID) 
 
def myCallback(buttons, event, timestamp):
    global robot
    if buttons & 0x02:
        robot.setLedColor(           #calls setLEDColor from 'barobo' module
            random.randint(0, 255),  #uses the 'random' integer generator from
            random.randint(0, 255),  #the random module to set colors
            random.randint(0, 255)
            )
 
robot.enableButtonEvents(myCallback)
 
input('Press "Enter" to quit.')

Callback Functions

Notice in the Linkbot program that we define a new function called "myCallback". However, you will notice in our code that we never actually call our function. The only other place it appears is inside a function called robot.enableButtonEvents(). The function myCallback is what is known as a "callback function". A callback function is a function that is written by a programmer, but the programmer doesn't actually call the function anywhere in their code. Instead, the programmer tells the computer program to call that function any time a certain event happens. In our case, by putting the function into the enableButtonEvents function, we are telling the program to call our callback function, myCallback, every time a button is pressed or released on the Linkbot.

The global keyword

Inside of the function myCallback, you will notice a line that reads global robot. The global keyword tells Python not to create a new variable, but instead use a variable in a higher scope that has already been declared. In this case, it tells Python to look for an existing variable called "robot" and use that.

You can only use the global keyword to find variables that are in an outer scope than where the global keyword is used. For instance, the following code is fine:

x = 5
 
def myFunc():
    global x
    print(x) # Prints the current value of x, which is 5
 
myFunc() # Call our function "myFunc"; should print "5"

However, This code will not work:

def myFunc():
    x = 5
 
def myOtherFunc():
    global x # This will not work!
    print(x)
 
myOtherFunc()

The previous code snippet will not work because the variable "x" is trapped inside the function myFunc(). Variables referred to with global need to be in the parent scope of where the global keyword is used.


Check out the range of colors and intensity the Linkbot is capable of generating with its built in RGB LED.

Exercises

Rewrite the high_low.py program from section Decisions to use an random integer between 0 and 99 instead of the hard-coded 78. Use the Python documentation to find an appropriate module and function to do this.

Solution

Rewrite the high_low.py program from section Decisions to use an random integer between 0 and 99 instead of the hard-coded 78. Use the Python documentation to find an appropriate module and function to do this.

from random import randint
number = randint(0, 99)
guess = -1
while guess != number: 
    guess = int(input ("Guess a number: "))
    if guess > number:
        print("Too high")
    elif guess < number:
            print("Too low")
print("Just right")


Learning Python 3 with the Linkbot
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