Summary  
This chapter covers using a loop with a counter (accumulator) variable to iteratively aggregate values from a collection.

General domain of usage  
Data aggregation from lists or arrays.

In programming, you often have to use a counter variable to perform basic arithmetic operations within a loop. This approach allows us to iteratively process data, such as summing values or tracking totals.

For example, if you want to calculate the sum of all numbers in a specific range, you can initialize a **counter** variable and update it during each iteration.

Let's adapt this concept to our common topic, working with the `travel_list`. Suppose you want to calculate the total length of all city names in our list.

travel_list = ['Monako', 'Luxemburg', 'Liverpool', 'Barcelona', 'Munchen']

# Initialize counter
total_length = 0

# Iteration through the list
for city in travel_list:
    # Add the length of each city name
    total_length += len(city)

print('Total length of all city names:', total_length)

The `+=` operator is a shorthand for addition and assignment. It adds the value on the right to the variable on the left and updates the variable. </br>For example, `x += 5` is equivalent to `x = x + 5`.

Note

import unittest
import importlib
import re

# Helper function for dynamic test naming and assertions
def _dynamic_test(test_case, condition, success_message, failure_message):
    if condition:
        test_case._testMethodName = success_message
        test_case.assertTrue(True, success_message)
    else:
        test_case._testMethodName = failure_message
        test_case.fail(failure_message)

EXPECTED_COUNTRIES = [
    'Wales', 'Denmark', 'Belgium', 'Japan', 'South Korea', 'South Africa', 'Indonesia',
    'Singapore', 'Australia', 'India', 'Saudi Arabia', 'Mexico', 'Turkey', 'Greece',
    'Netherlands', 'Finland', 'Monako', 'United Arab Emirates', 'Egypt', 'Morocco',
    'Brazil', 'Argentina', 'Ireland', 'Portugal', 'Chile', 'Spain', 'Czech Republic',
    'Sweden', 'Switzerland', 'Thailand', 'Luxemburg', 'New Zealand', 'France', 'Italy',
    'Germany', 'China', 'Canada', 'Hungary', 'Scotland', 'Norway', 'Austria', 'Ukraine',
    'Poland'
]

class TestUserCode(unittest.TestCase):
    @classmethod
    def setUpClass(cls):
        cls.user_code = importlib.import_module("user_code")
        with open("user_code.py", "r", encoding="utf-8") as f:
            cls.source = f.read()

    def test_counter_is_declared(self):
        _dynamic_test(
            self,
            hasattr(self.user_code, "counter"),
            "The `counter` variable is declared.",
            "Expected `counter` to be declared."
        )

    def test_counter_is_integer(self):
        _dynamic_test(
            self,
            isinstance(getattr(self.user_code, "counter", None), int),
            "The `counter` variable is an integer.",
            "`counter` should be an integer."
        )

    def test_countries_is_declared_and_is_list(self):
        _dynamic_test(
            self,
            hasattr(self.user_code, "countries") and isinstance(self.user_code.countries, list),
            "The `countries` variable is declared and is a list.",
            "Expected `countries` to be declared and be a list."
        )

    def test_countries_list_matches_expected(self):
        _dynamic_test(
            self,
            getattr(self.user_code, "countries", None) == EXPECTED_COUNTRIES,
            "The `countries` list matches the expected destinations.",
            f"Expected `countries` to be `{EXPECTED_COUNTRIES}`, but got `{getattr(self.user_code, 'countries', None)}`."
        )

    def test_for_loop_used_over_countries(self):
        """Check if a `for` loop iterates over `countries` (robust AST-based check)."""
        import ast
    
        try:
            tree = ast.parse(self.source)
        except SyntaxError:
            _dynamic_test(
                self,
                False,
                "",
                "Could not parse `user_code.py` to check for a `for` loop."
            )
            return
    
        def iter_uses_countries(iter_node):
            # Accepts: countries
            if isinstance(iter_node, ast.Name) and iter_node.id == "countries":
                return True
    
            # Accepts: list(countries), tuple(countries), set(countries)
            if isinstance(iter_node, ast.Call) and isinstance(iter_node.func, ast.Name):
                if iter_node.func.id in {"list", "tuple", "set"} and iter_node.args:
                    return iter_uses_countries(iter_node.args[0])
    
            # Accepts: countries[...], countries[::2], etc.
            if isinstance(iter_node, ast.Subscript):
                return iter_uses_countries(iter_node.value)
    
            # Accepts: (countries), or other wrappers
            if isinstance(iter_node, ast.Attribute):
                # not expected here, but keep conservative
                return False
    
            return False
    
        found = False
        for node in ast.walk(tree):
            if isinstance(node, ast.For) and iter_uses_countries(node.iter):
                found = True
                break
    
        _dynamic_test(
            self,
            found,
            "A `for` loop iterating over `countries` is used in the code.",
            "Expected a `for` loop iterating over `countries` (directly or via slicing/casting), but none was found."
        )


    def test_len_check_equals_6_used(self):
        # Accepts either: len(country) == 6  OR  6 == len(country)
        has_len_eq_6 = (
            re.search(r"len\s*\(\s*\w+\s*\)\s*==\s*6", self.source) is not None
            or re.search(r"6\s*==\s*len\s*\(\s*\w+\s*\)", self.source) is not None
        )
        _dynamic_test(
            self,
            has_len_eq_6,
            "The code checks for country names with exactly 6 letters using `len(...) == 6`.",
            "Expected a check like `len(country) == 6` inside the loop, but it was not found."
        )

    def test_counter_result_is_correct(self):
        expected_result = sum(1 for c in EXPECTED_COUNTRIES if len(c) == 6)
        _dynamic_test(
            self,
            getattr(self.user_code, "counter", None) == expected_result,
            f"The result is correct: {expected_result} countries have exactly 6 letters.",
            f"Expected {expected_result} countries with 6 letters, but found {getattr(self.user_code, 'counter', None)}."
        )

if __name__ == "__main__":
    unittest.main()


test_main.py

Build confidence in mastering Python loops! Start with the basics of for and while loops and progress to advanced concepts like nested loops, control statements, and dynamic conditions. With hands-on practice and real-world examples, gain the skills to write efficient, clean, and powerful code. Perfect for beginners and anyone looking to sharpen their Python expertise!

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Counting with Loops

Solution