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Functional and procedural approach Python: Declarative programming

Python's creators wanted its users to actively and effectively use built-in collections.

Therefore, the language inclines the programmer toward a particular work style when choosing an approach. The language itself encourages the use of an imperative style combined with procedural and object-oriented programming.

In Python, collections are objects. We modify them mostly in place, which gives them their modifiable state. However, the language also has a place for elements of functional programming. Developers often consider it a subspecies of declarative programming because functional code looks like a data conversion pipeline.

In this lesson, we'll look at imperative and functional code in Python. In both cases, we'll try displaying a sorted list of unique elements from a set of numbers.

Procedural solution

When we use procedural solutions, we still split the program into subroutines that change the state instead of returning a value. The code will look like this:

INPUT = [1, 2, 5, 12, 3, 5, 2, 7, 12]

def main():
    numbers = INPUT[:]
    filter_and_sort(numbers)  # Sorting and filtering in place
    for number in numbers:
        print(number)         # Outputting element by element in a loop

def filter_and_sort(values):
    values.sort()  # The list is sorted in place
    previous_value = None
    index = 0
    while index < len(values):
        value = values[index]
        if value == previous_value and index > 0:
            # We remove the item from the list, modifying the list again
            index += 1

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