MacLochlainns Weblog

It seems a number of my students had some confusion over how to write overloaded Python functions. So, I prepared this little tutorial using Python 3.

The first basic1.py example file is a standalone Python file that:

• Defines a hello() world function.
• Calls the local hello() world function.

#!/usr/bin/python
 
# Define a hello() function.
def hello():
  print("Hello World!")
 
# Call the hello() function.
hello()

You can test the basic1.py script as follows:

./basic1.py

It prints:

Hello World!

The second basic2.py example file is also a standalone Python file that:

• Attempts to define overloaded hello() world functions. One version takes no arguments and the other takes one argument.
• Attempts to call the overloaded local hello() world function without any arguments and with one argument.

#!/usr/bin/python
 
# Call the hello() function without any arguments.
def hello():
  print("Hello World!")
 
# Call the hello() function with one argument.
def hello(whom):
  print("Hello", whom)
 
# Call the overloaded hello() functions.
hello()
hello("Henry")

You can test the basic2.py script as follows:

./basic2.py

It successfully defines the hello() function and then it replaces it with the hello(whom) function. So, it raises the following runtime error because the call to the hello() world function finds the hello(whom) function and the call lacks a call parameter.

Traceback (most recent call last):
  File "/home/student/Code/python/funct/./basic2.py", line 12, in <module>
    hello()
TypeError: hello() missing 1 required positional argument: 'whom'

The third basic3.py example file is also a standalone Python file that:

• Defines a function that acts like an overloaded hello(whom=None) world function.
• Call the local hello(whom=None) world function without any arguments and with one argument. It works because you do two things:
  • You assign a default null value to the whom parameter, which makes the parameter optional in the function’s signature.
  • You use an if-statement to manage the behavior of a null parameter. The None keyword defines a null value. Please note that the is reference comparison operator is necessary to evaluate whether a variable contains a null value.

#!/usr/bin/python
 
# Call the hello() function with an optional parameter; and
# manage the inner workings with and without a parameter.
def hello(whom = None):
  if whom is None:
    print("Hello World!")
  else:
    print("Hello", whom + "!")
 
# Call the overloaded hello() functions.
hello()
hello("Henry")

You can test the basic3.py script as follows:

./basic3.py

It prints:

Hello World!
Hello Henry!

At this point, we need to qualify how you can position a Python library file in a development directory. Development directories aren’t typically in the standard library locations, which means you need to define the directories in the $PYTHONPATH environment variable.

There’s a convenient trick that lets you set the $PYTHONPATH value so that you can use it across multiple test environments. It requires you to create an src directory for your library source code inside the directory where you develop code that will use library functions.

After creating the src directory, you can set the $PYTHONPATH environment variable with a relative src directory in the following syntax:

export set PYTHONPATH=$PYTHONPATH:./src:.

It will now let Python look for libraries in the src subdirectory or the present working directory.

Next, you deploy the following hello(whom=None) function in a lib.py file inside the src subdirectory.

# Call the hello() function with an optional parameter; and
# manage the inner workings with and without a parameter.
def hello(whom = None):
  if whom is None:
    print("Hello World!")
  else:
    print("Hello", whom + "!")

In the parent directory of the src subdirectory create the basic4.py file, like:

#!/usr/bin/python
 
# Import the lib.py file as a lib package.
import lib
 
# Call the hello() function without arguments and
# with one argument within the namespace of the lib 
# library.
lib.hello()
lib.hello("Henry")

An alternate way to write the basic4.py program imports a single namespace element (like a variable, function, or object) and places it in the local namespace of the program. You can redefine hello() function

#!/usr/bin/python
 
# Define the hello namespace element from the lib 
# library in the current program.
from lib import hello
 
# Call the hello() function without arguments and
# with one argument within the namespace of the lib
# library.
hello()
hello("Henry")

The hello() function only prints messages. You can add a return statement to return a value from the hello() function. The modified library returns a string rather than printing a string, as follows:

# Call the hello() function with an optional parameter; and
# manage the inner workings with and without a parameter.
def hello(whom = None):
  if whom is None:
    return "Hello World!"
  else:
    return "Hello " + whom + "!"
 
# Call the goodbye() function with an optional parameter; and
# manage the inner workings with and without a parameter.
def goodbye(whom = None):
  if whom is None:
    return "Goodbye World!"
  else:
    return "Goodbye " + whom + "!"

You would then make the following changes to the basic5.py program that calls the lib.py library file. You could also call the goodbye() function inside the imported lib scope. However, you wouldn’t be able to call the goodbye() function if you had imported only the hello() function from the lib package into the local namespace.

#!/usr/bin/python
 
# Import the lib.py file as a lib package.
import lib
 
# Call the hello() function without arguments and
# with one argument within the namespace of the lib
# library.
print(lib.hello())
print(lib.hello("Henry"))

As always, I hope this helps those looking to understand and use functions in Python.