Programming paradigms

What is a paradigm?

• A programming paradigm is a style of organising a program.
• The same problem can be solved in different styles.
• Python lets you mix several paradigms in one program.

Imperative and procedural

• Imperative code is a list of steps that change the program's state.
• It says how to do something, step by step (loops, assignments).
• Procedural style groups those steps into reusable procedures and functions.

def total(nums):
    result = 0
    for n in nums:
        result = result + n
    return result

print(total([2, 4, 6]))

Object-oriented

• Object-oriented code bundles data and the methods that act on it into objects.
• You model the problem as a set of classes that talk to each other.
• It suits large programs because each class is a self-contained part.

class Counter:
    def __init__(self):
        self.count = 0
    def add(self):
        self.count = self.count + 1

c = Counter()
c.add()
c.add()
print(c.count)

Declarative

• Declarative code says what you want, not how to compute it.
• The language or library works out the steps for you.
• Examples: a database query (SQL), or a Python list comprehension.

nums = [1, 2, 3, 4, 5]
print(sum(nums))                 # say WHAT: the total
print([n * n for n in nums])     # a list comprehension

Low-level

• Low-level code is close to the machine: assembly and machine code.
• The CPU runs it directly, so it is fast but hard to write.
• High-level languages like Python are translated down to it before running.

Common mistakes

• Procedural is steps; object-oriented is objects with data and methods; declarative describes the result.
• Match the paradigm to the problem.

Now you try

• Each task solves a small problem in a different paradigm.
• Press Check answer to test your code.