In the vast and exciting world of programming, there are concepts that are fundamental pillars for any developer, regardless of their experience level. One of these concepts is functions. What are they? Why are they so crucial? Let's find out!

What are functions?

A function, in simple terms, is a block of code that executes only when called. You can think of it as a small program within your main program, designed to perform a specific task1. A function can also be seen as a black box: we pass an input (parameters), some internal processing occurs, and it produces an output (return value).

Functions allow us to segment our code into logical parts where each part performs a single action. This provides several benefits2:

  • Reusability: Once defined, we can execute (call) that code from anywhere in our program as many times as needed.
  • Organization: It allows dividing a large program into smaller, more manageable parts.
  • Encapsulation: Functions reduce complexity by hiding internal implementation details.
  • Maintainability: If we need to make changes, we only have to modify the code in one place (the function) instead of tracking down all instances of that code.

Procedures vs. Functions

It is vital to distinguish between these two concepts. While a function always returns a value, a procedure performs a task but does not return anything. In some languages, this difference is clearer than in others. Python, for example, has functions that can optionally return values.

Anatomy of a function

In Python, a function is declared using the def keyword, followed by the function name and parentheses. The code inside the function is called the body of the function3 and contains the set of instructions to execute to perform its task.

1def my_function():
2    print("Hello from my function!")

To call or invoke a function, we simply use its name followed by parentheses:

1my_function() # Output: Hello from my function!

Parameters and arguments

Functions become even more powerful when we pass information to them, known as parameters. These act as “variables” inside the function, allowing the function to work with different data each time it is called.

While parameters are variables defined in the function definition, arguments are the actual values passed when calling the function.

1def greet(name):
2    print(f"Hello {name}!")
5# Output:
6#   Hello Maria!

Python allows default parameters, which have a predetermined value, making passing those arguments optional when calling the function. It also allows named parameters which enable passing arguments in any order by specifying their name.

 1def greet(name="Maria", repetitions=3):
 2    repetition = 1
 3    while repetition <= repetitions:
 4        print(f"Hello {name}!")
 5        repetition += 1
 8# Output:
 9#   Hello Maria!
10#   Hello Maria!
11#   Hello Maria!
13greet("Florencia", 4)
14# Output:
15#   Hello Florencia!
16#   Hello Florencia!
17#   Hello Florencia!
18#   Hello Florencia!
20greet(repetitions=2, name="Julian")
21# Output
22#   Hello Julian!
23#   Hello Julian!

Returning values

Functions can return a result or return value using the return keyword.

1def circle_area(radius):
2    return 3.14 * (radius ** 2)
4result = circle_area(10)
5print(result) # Output: 314

The return value is passed back to where the function was called and can be assigned to a variable for later use.

Functions can also perform some task without explicitly returning anything. In Python this is known as returning None.

Local and global variables

Local variables are defined inside a function and only exist in that scope, while global variables are defined outside and can be accessed from anywhere in the code. It is crucial to understand their scope (where a variable is accessible) and lifetime (how long a variable lives).

1x = 10 # x is global
3def add():
4    y = 5 # y is local
5    return x + y
7add() # Output: 15
8print(y) # Error, y does not exist outside the function

We can read global variables from a function, but if we need to modify it we must declare it global.

1x = 10
3def add():
4    global x
5    x = x + 5
8print(x) # 15

Best Practices

When creating functions we should follow certain principles and patterns4:

  • The name of a function should clearly indicate its purpose.
  • Make functions small, simple, and focused on one task. A function should do one thing and do it well.
  • Use descriptive names for functions and parameters.
  • Avoid side effects and modifying global variables.
  • Properly document the purpose and usage of each function.
  • Limit the number of parameters, ideally 0 to 3 parameters.

Following these best practices will help us create reusable, encapsulated, and maintainable functions.


Functions are core components in programming, allowing us to organize, reuse, and encapsulate code. By defining functions that perform a single task we keep our programs simplified, easy to understand, and modify. By understanding and mastering this concept, you not only improve the quality of your code but also your efficiency as a developer.


  1. McConnell, S. (2004). Code Complete. Microsoft Press. ↩︎

  2. Joyanes Aguilar, L. (2008). Fundamentos de programación: algoritmos, estructura de datos y objetos. McGraw-Hill. ↩︎

  3. Python Software Foundation. (2022). Python Official Documentation↩︎

  4. Kindler, E., & Krivy, I. (2011). Object-Oriented Simulation of systems with Java: A working introduction. BoD–Books on Demand. ↩︎