Introduction to functions and modules

Functions allow you to:

  • Break big problems into smaller, manageable pieces
  • Reuse code without copying and pasting
  • Test parts of your program independently and debug it more easily

Modules allow you to:

  • Keep your code organized and manageable
  • Reuse code across different projects
  • Share code with others

What is a function?

A function is a named block of code that performs a specific task. Functions help you organize your code, avoid repetition, and make your programs easier to read and maintain (you can fix a mistake once and it will fix it wherever the function is invoked).

Example:

# Feel free to play around with this code # Define the function def greet_user(): print(f"Hello!") greet_user() # Call the function 





Defining and calling functions



    
  
  • Use the def keyword to define a function
    • 
  
  • Give your function a name (by convention use lowercase and underscores)
    • 
  
  • Use parentheses () for parameters (input variables)
    • 
  
  • End the definition with a colon :
    • 
  
  • Indent the code inside the function
    • 




Example:




  

# Feel free to play around with this code

def add(a, b):
    return a + b

result = add(3, 5) 
print(result)  


  
  





Parameters and arguments



# Declare function add, which takes in two parameters, a and b
def add(a, b):
    return a + b

result = add(3, 5) # When calling the function, the parameters now have values
print(result)  # Output: 8




    
  
  • Parameters are variables listed in the function definition (a, b above).
    • 
  
  • Arguments are the actual values you pass when calling the function (3, 5 above).
    • 




Return values



    
  
  • Use return to send a value back from the function.
    • 
  
  • If you don’t use return, the function returns None by default.
    • 




Example:


def square(x):
    return x * x

print(square(4))  # Output: 16



This gives the output:


16




But this:


def square(x):
    x * x

print(square(4)) # Output: None



gives the output:


None







What is a module?



A module is a file containing Python code (functions, variables, classes) that you can use in other programs. Modules help you organize large projects by splitting code into multiple files.



Modules allow you to:


    
  
  • Keep your code organized and manageable
    • 
  
  • Reuse code across different projects
    • 
  
  • Share code with others
    • 




Example:
Suppose you have a file called calculator.py:


# calculator.py

def square(x):
    return x * x



This file can be accessed by another python file as the calculator module.



import calculator

print(calculator.square(4)) # Output: 16



This alls the square(x) function declared in calculator.py using the full reference preceded by calculator: calculator.square().



Importing modules



There are three ways to import a module and its functions:


    
  
  • Use import module_name to bring in a module. Then to call a function, you use module_name.function_name()
    • 
  
  • Use import module_name as alias to give a module a different name (often something shorter or more understandable). Then to call the function, you use alias.function_name()
    • 
  
  • Use from module_name import function_name to import specific functions. Then to call the function, you simply use function_name()
    • 
  
  • Use from module_name import function_name as alias to import specific functions and give them a different name
    • 




For example, to use the square(x) function declared in calculator.py, you can either use:



import calculator
print(calculator.square(4)) # Output: 16



or with an alias:


import calculator as calc
print(calc.square(4)) # Output: 16



or with the specific function imported, so you can simply use square()


from calculator import square
print(square(4)) # Output: 16



or with a specific function alias


from calculator import square as sq
print(sq(4))  # Output: 16




Using third-party libraries



A library is a collection of modules written by others that you can use in your projects. Importing a library uses the same syntax as importing a module.



For example, this code imports a third-party library called tkinter, a library for building graphical user interfaces (GUIs)



Example:


# Both use the same import syntax
import calculator        # Import a single module (one file)
import tkinter          # Import a library (many files/modules)

window = tk.Tk()
window.title("My App")
window.mainloop()




A library is essentially a folder containing multiple modules:


tkinter/              # Library (folder)
├── __init__.py      # Makes it a package
├── dialog.py        # Module within the library
├── font.py          # Another module
└── messagebox.py    # Another module




As well as importing the whole library (as above), you can also import specific modules from it:



from tkinter import messagebox