Summary  
Nested loops enable one loop to run inside another, allowing iteration over multiple sequences or dimensions—useful for combining elements from different collections or traversing nested data structures.  

General domain of usage  
Inventory management

Nested loops **extend the power of simple loops** by allowing you to perform repeated actions **across multiple dimensions**. This is crucial for handling more complex tasks in a grocery store environment, such as organizing **multi-layered inventory data** or coordinating schedules across different departments.

Watch as Alex showcases how nested loops can be utilized to enhance efficiency and manage complex scenarios in our grocery store setting:


Nested loops are **incredibly useful** in programming when you need to **loop through multiple sequences** at the same time. By placing one loop inside another, you can iterate over elements in two or more dimensions, such as a list of lists: 


# List of lists representing stock in different departments
department_stocks = [
    ["Apples", "Bananas", "Cherries"],  # Fruits
    ["Milk", "Cheese", "Butter"],       # Dairy
    ["Bread", "Bagels", "Muffins"]      # Bakery
]

print("Inventory Check:")
for department in department_stocks:
    print(department) # For each iteration of the outer loop, the entire sublist is accessed
    # The inner loop then iterates over the items in that sublist
    for item in department:
        print(f" - {item}")
    print("")  # Add a line break for clarity

Notice how the **outer loop iterates through each sublist** in `department_stocks`, where each sublist represents a different department. The **inner loop then iterates through all the items** within each sublist. The outer loop does not proceed to the next sublist until the inner loop has finished iterating through all items in the current sublist.


import unittest
import importlib
import io
from contextlib import redirect_stdout

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)

class TestSimpleNestedLists(unittest.TestCase):
    def test_groceries_structure(self):
        import user_code
        importlib.reload(user_code)
        groceries = getattr(user_code, 'groceries', None)
        expected = [["Tomatoes", "Lettuce"], ["Milk", "Cheese"]]
        _dynamic_test(
            self,
            groceries == expected,
            "groceries is correctly combined",
            f"Expected groceries = {expected}, but got {groceries}"
        )
        
    def test_output_items(self):
        import user_code
        importlib.reload(user_code)
        f = io.StringIO()
        with redirect_stdout(f):
            importlib.reload(user_code)
        output = f.getvalue().strip().splitlines()
        expected = ["Tomatoes", "Lettuce", "Milk", "Cheese"]
        found_all = all(any(word in line for line in output) for word in expected)
        _dynamic_test(
            self,
            found_all,
            "All expected item names are present in the output lines",
            f"Not all expected item names found in output lines. Expected words: {expected}, got lines: {output}"
        )
         
    def test_nested_loops_used(self):
        import user_code
        import inspect
        source = inspect.getsource(user_code)
        for_count = source.count('for')
        _dynamic_test(
            self,
            for_count >= 2,
            "At least two for loops (nested) are used in the code",
            f"Expected at least two for loops, found {for_count}"
        )
if __name__ == '__main__':
    unittest.main()


test_main.py

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Nested Loops

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