MacLochlainns Weblog

I promised to give my students a full example of how to write and execute a Python object. There were two motivations for this post. The first was driven by my students trying to understand the basics and the second my somebody else saying Python couldn’t deliver objects. Hopefully, this code is simple enough for both audiences. I gave them this other tutorial on writing and mimicking overloaded Python functions earlier.

This defines a Ball object type and a FilledBall object subtype of Ball. It incorporates the following elements:

A special __init__ function, which is a C/C++ equivalent to a constructor.
A special __str__ function represents a class object instance as a string. It is like the getString() equivalent in the Java programming language.
A bounce instance function, which means it acts on any instance of the Ball object type or FilledBall object subtype.
A get_direction instance function and it calls the __format local object function, which is intended to mimic a private function call, like other object-oriented programming languages.
A private name __format function (Private name mangling: When an identifier that textually occurs in a class definition begins with two or more underscore characters and does not end in two or more underscores, it is considered a private name of that class.)

You can test this code by creating the $PYTHONPATH (Unix or Linux) or %PYTHONPATH% (Windows) as follows with all the code inside the present working directory, like this in Unix or Linux:

export set $PYTHONPATH=.

Then, you create the Ball.py file with this syntax:

# Creates a Ball object type and FilledBall object subtype.
class Ball:
  # User-defined constructor with required parameters.
  def __init__(self, color = None, radius = None, direction = None):
    # Assign a default color value when the parameter is null.
    if color is None:
      self.color = "Blue"
    else:
      self.color = color.lower()
 
    # Assign a default radius value when the parameter is null.
    if radius is None:
      self.radius = 1
    else:
      self.radius = radius
 
    # Assign a default direction value when the parameter is null.
    if direction is None:
      self.direction = "down"
    else:
      self.direction = direction.lower()
 
    # Set direction switch values.
    self.directions = ("down","up")
 
  # User-defined standard function when printing an object type.
  def __str__(self):
    # Build a default descriptive message of the object.
    msg = "It's a " + self.color + " " + str(self.radius) + '"' + " ball"
 
    # Return the message variable.
    return msg
 
  # Define a bounce function.
  def bounce(self, direction = None):
    # Set direction on bounce.
    if not direction is None:
      self.direction = direction
    else:
      # Switch directions.
      if self.directions[0] == self.direction:
        self.direction = self.directions[1]
      elif self.directions[1] == self.direction:
        self.direction = self.directions[0]
 
  # Define a bounce function.
  def getDirection(self):
    # Return current direction of ball.
    return self.__format(self.direction)
 
  # User-defined pseudo-private function, which is available
  # to instances of the Ball object and any of its subtypes.
  def __format(self, msg):
    return "[" + msg + "]"
 
# This is the object subtype, which takes the parent class as an
# argument.
class FilledBall(Ball):
  def __init__(self, filler = None):
    # Instantiate the parent class and then any incremental 
    # parameter values.
    Ball.__init__(self,"Red",2)
 
    # Add a default value or the constructor filler value.
    if filler is None:
      self.filler = "Air".lower()
    else:
      self.filler = filler
 
  # User-defined standard function when printing an object type, which 
  # uses generalized invocation.
  def __str__(self):
    # Build a default descriptive message of the object.
    msg = Ball.__str__(self) + str(" filled with " + self.filler)
 
    # Return the message variable.
    return msg

# Creates a Ball object type and FilledBall object subtype. class Ball: # User-defined constructor with required parameters. def __init__(self, color = None, radius = None, direction = None): # Assign a default color value when the parameter is null. if color is None: self.color = "Blue" else: self.color = color.lower() # Assign a default radius value when the parameter is null. if radius is None: self.radius = 1 else: self.radius = radius # Assign a default direction value when the parameter is null. if direction is None: self.direction = "down" else: self.direction = direction.lower() # Set direction switch values. self.directions = ("down","up") # User-defined standard function when printing an object type. def __str__(self): # Build a default descriptive message of the object. msg = "It's a " + self.color + " " + str(self.radius) + '"' + " ball" # Return the message variable. return msg # Define a bounce function. def bounce(self, direction = None): # Set direction on bounce. if not direction is None: self.direction = direction else: # Switch directions. if self.directions[0] == self.direction: self.direction = self.directions[1] elif self.directions[1] == self.direction: self.direction = self.directions[0] # Define a bounce function. def getDirection(self): # Return current direction of ball. return self.__format(self.direction) # User-defined pseudo-private function, which is available # to instances of the Ball object and any of its subtypes. def __format(self, msg): return "[" + msg + "]" # This is the object subtype, which takes the parent class as an # argument. class FilledBall(Ball): def __init__(self, filler = None): # Instantiate the parent class and then any incremental # parameter values. Ball.__init__(self,"Red",2) # Add a default value or the constructor filler value. if filler is None: self.filler = "Air".lower() else: self.filler = filler # User-defined standard function when printing an object type, which # uses generalized invocation. def __str__(self): # Build a default descriptive message of the object. msg = Ball.__str__(self) + str(" filled with " + self.filler) # Return the message variable. return msg

Next, let’s test instantiating the Ball object type with the following code:

#!/usr/bin/python
 
# Import the Ball class into its own namespace.
import Ball
 
# Assign an instantiated class to a local variable.
myBall = Ball.Ball()
 
# Check whether the local variable holds a valid Ball instance.
if not myBall is None:
  print(myBall, "instance.")
else:
  print("No Ball instance.")
 
# Loop through 10 times changing bounce direction.
for i in range(1,10):
  # Find dirction of ball.
  print(myBall.getDirection())
 
  # Bounce the ball.
  myBall.bounce()

Next, let’s test instantiating the FilledBall object subtype with the following code:

#!/usr/bin/python
 
# Import the Ball class into its own namespace.
import Ball
 
# Assign an instantiated class to a local variable.
myBall = Ball.FilledBall()
 
# Check whether the local variable holds a valid FilledBall instance.
if not myBall is None:
  print(myBall, "instance.")
else:
  print("No Ball instance.")
 
# Loop through 10 times changing bounce direction.
for i in range(1,10):
  # Find dirction of ball.
  print(myBall.getDirection())
 
  # Bounce the ball.
  myBall.bounce()

As always, I hope this helps those looking to learn and extend their knowledge.

The following are samples of creating, changing, and removing users and groups in Linux. These are the command-line options in the event you don’t have access to the GUI tools.

Managing Users:

Adding a user:

The prototype is:

# useradd [-u uid] [-g initial_group] [-G group[,...]] \
 > [-d home_directory] [-s shell] [-c comment] \
 > [-m [-k skeleton_directory]] [-f inactive_time] \
 > [-e expire_date] -n username