Functions & Scope

What is a Function?

In mathematics, a function maps inputs to outputs: f(x) = y. In programming, a function is a named, reusable block of code that accepts zero or more inputs (called parameters), executes a defined task, and optionally returns a value. Functions are the primary mechanism for abstraction and code reuse — the industry principle called DRY (Don't Repeat Yourself) says: if you write the same logic twice, it belongs in a function.

Functions also make code testable and readable: instead of a 200-line script, well-written code reads like a sequence of named steps.

Basic Syntax

def calculate_mse(actual, predicted):
    n = len(actual)
    error = sum((a - p)**2 for a, p in zip(actual, predicted)) / n
    return error

Default & Keyword Arguments

def train_model(data, epochs=10, learning_rate=0.01):
    pass

train_model(my_data)                          # uses defaults
train_model(my_data, epochs=50, learning_rate=0.1) # overrides

*args and **kwargs

Pass a variable number of arguments:

def log_metrics(**kwargs):
    for key, val in kwargs.items():
        print(f"{key}: {val}")
        
log_metrics(accuracy=0.9, loss=0.05)