Summary  
This chapter covers using nested for loops to control multi-level iteration, enabling structured printing patterns and element-wise processing of nested lists.  

General domain of usage  
Processing nested data structures

A **nested loop** is a loop that operates within the body of another loop. In this structure, the inner loop executes completely for each iteration of the outer loop. You can combine different types of loops (e.g., a `while` loop with a `for` loop) to achieve various results.

Definition

We will use a nested loop to print each city multiple times in a structured format, resembling a **triangle pattern**.

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

# Outer loop for controlling rows
for i in range(1, len(travel_list) + 1):
    # Inner loop for controlling columns
    for j in range(i):
        print(travel_list[j], end=' ')  # Print cities in a row
    print('')  # Move to the next line after each row

- **Outer loop**:
the `for` loop with `range(1, len(travel_list) + 1)` determines the **number of rows**. Each iteration represents a row. `i` controls how many cities are printed in the current row;
- **Inner loop:**
the `for` loop with `range(i)` iterates over the cities to be printed in the current row.
`j` accesses the city names in the `travel_list` up to the current row index;
- **Printing:**
the `print(travel_list[j], end=' ')` ensures cities are printed on the same row.
`print('')` moves to the next line after each row.

Using a nested loop, you can iterate through a nested list and interact with each of its elements.

# Define a nested list containing sublists with words starting with 'A' and 'T'
nested_list = [
    ["Apple", "Avocado", "Apricot"],
    ["Tomato", "Tangerine", "Tea"],
    ["Almond", "Thyme", "Tuna"]
]

# Iterate through each sublist in the nested list
for sublist in nested_list:
    # Iterate through each item in the current sublist
    for item in sublist:
        # Convert the item to lowercase and print it
        print(item.lower(), end=' ')

import unittest
import importlib

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 TestUserCode(unittest.TestCase):

    def test_countries_is_declared(self):
        try:
            user_code = importlib.import_module("user_code")
            _ = user_code.countries
            _dynamic_test(self, True, "The `countries` list is declared.", "")
        except AttributeError:
            _dynamic_test(self, False, "", "Expected `countries` to be declared.")
        except ImportError as e:
            self.fail(f"Failed to import module: {e}")

    def test_countries_contains_correct_values(self):
        try:
            user_code = importlib.import_module("user_code")
            expected_countries = sorted({country.capitalize() for trip in user_code.trips for country in trip})
            _dynamic_test(
                self,
                sorted(user_code.countries) == expected_countries,
                "The `countries` list contains all countries from `trips` with correct capitalization.",
                f"Expected `countries` to be `{expected_countries}`, but got `{user_code.countries}`."
            )
        except AttributeError:
            _dynamic_test(self, False, "", "Expected `countries` to be declared.")
        except ImportError as e:
            self.fail(f"Failed to import module: {e}")

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

test_main.py

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Nested for Loop

解答