West Midlands | 26 March SDC | Iswat Bello | Sprint 2 | Implement a linked list in python by Iswanna · Pull Request #191 · CodeYourFuture/Module-Complexity

Iswanna · 2026-06-30T12:50:07Z

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My changes meet the requirements of the task
I have tested my changes
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Changelist

In this PR, I implemented a Doubly Linked List to handle data storage and removal efficiently. A detailed breakdown of the implementation and trade-offs can be found in the CHANGES-MADE.md file included in this submission.

Key Changes

Node & LinkedList Classes: Defined a Node class with bidirectional pointers (next and previous) and a LinkedList class to manage the head and tail.
Method Implementation:
- push_head(value): Adds a node to the front in O(1) time.
- pop_tail(): Removes the last node in O(1) time, including logic to handle single-node and empty lists.
- remove(node): A robust method that re-links neighboring nodes to remove a specific entry from anywhere in the list.
Pointer Management: Ensured that all pointers (head, tail, next, and previous) are correctly updated during every operation to maintain list integrity.

Testing Done

I verified the code using unittest with the following scenarios:

Standard push_head and pop_tail sequences.
Middle Removal: Verified that removing a center node correctly re-wires the nodes ahead and behind it.
Empty List Handling: Confirmed that pop_tail returns None rather than crashing when the list is empty.
Null Safety: Confirmed that remove(None) is handled gracefully.

Learning Reflection (Trade-offs)

I applied a space-vs-time trade-off by using extra memory (for the previous pointer) to ensure that adding to the head and removing from the tail stays at O(1) speed regardless of how many items are in the list.

- Define Node class to store value - Add pointers for next and previous nodes

- Define the LinkedList class in linked_list.py. - Implement the __init__ method to track the head and tail of the list. - Initialize head and tail to None for an empty list state.

- Added the logic to create a new node and insert it at the front of the list. - Handled pointer updates for both empty and non-empty list states. - Returns the new node to allow for future removal/referencing.

- Added logic to remove the last node from the list. - Handles edge cases for empty lists and single-node lists. - Updates tail pointers and returns the value of the removed node.

- Added logic to remove a specific node from any position. - Included pointer updates for head and tail edge cases. - Added neighbor re-linking to maintain list integrity.

- Added test_remove_middle to verify pointer re-linking when deleting from the center. - Added test_pop_empty_list to ensure the program doesn't crash on empty lists. - Added test_remove_none to verify handling of null input.

- Documented core logic for Node and LinkedList classes - Explained testing strategy for edge cases (empty lists, middle removal) - Traded extra memory space to achieve O(1) time (constant speed).

- Changed "Linked List" to "Doubly Linked List" to accurately reflect the bi-directional implementation. - Updated header for better technical clarity.

cjyuan · 2026-07-02T14:24:52Z

+    """
+    Represents a single person in line.
+    Stores a value and links to the people ahead and behind.
+    """
+    def __init__(self, value):
+        self.value = value
+        self.next = None      # The person ahead
+        self.previous = None  # The person behind


May I suggest exploring the use of __slots__ to reduce memory usage?

Thank you for the suggestion. I have implemented __slots__ to reduce memory usage.

cjyuan · 2026-07-02T14:36:02Z

+        # Logic 3: If the list is not empty
+        else:
+            new_node.next = self.head      # New person grabs old head's hand
+            self.head.previous = new_node  # Old head reaches back to new person
+            self.head = new_node           # New person is now the head


else already means "opposite", so you could probably drop the comment on line 28.

The comments on line 30-32 actually confuses me because it refers to "person". Are they your own notes? If so, it might not be a good idea to leave them in "production" code.

Thank you for the feedback.
Those metaphors were part of my initial notes to help me understand the concept. I've cleaned up the code to remove the non-technical language and the redundant logic labels.

cjyuan · 2026-07-02T15:18:00Z

+
+    def push_head(self, value):


Could consider adding type hints to each method to clearly specify the types of its parameters and return value.

Thank you for the feedback. I have updated the code to implement your suggestions.

cjyuan · 2026-07-02T15:19:08Z

+        # Logic 2: If there is only one person in line
+        if self.head == self.tail:
+            self.head = None
+            self.tail = None
+        # Logic 3: If there are more people
+        else:
+            self.tail = self.tail.previous  # Move tail back one
+            self.tail.next = None           # New tail lets go of the old tail


Why not just call remove()?

Thank you for the suggestion. I’ve refactored the pop_tail method to call self.remove(self.tail)

cjyuan · 2026-07-02T15:30:14Z

+        # Logic 1: Store the value to return later
+        value_to_return = self.tail.value


The variable name is self-explanatory. The comment does not provide any more info.

I have removed all redundant comments. Thank you.

cjyuan · 2026-07-02T15:31:09Z

+        # Logic 2: If there is only one person in line
+        if self.head == self.tail:
+            self.head = None
+            self.tail = None
+        # Logic 3: If there are more people


What do these "logic numbers" represent?

They represented the steps of the logic, but I have now removed them.

- Add __slots__ to prevent per-instance dictionary overhead - Optimize space complexity for large-scale list usage

- Remove numbered logic comments (# Logic 1, 2, 3) to reduce noise - Remove non-technical metaphors (e.g., "New person") in favor of self-documenting code - Clean up method bodies to follow professional clean code standards

- Implement typing (Any, Optional) for parameters and return values - Add forward references for Node pointers

- Replace manual pointer logic in pop_tail with a call to self.remove() - Adhere to DRY (Don't Repeat Yourself) principles - Reduce code duplication and improve maintainability

cjyuan

Changes look great. Code is very clean!

If you want to enforce that a linked list can only contain values of a single type (e.g. all ints or all strs), have a look at Python's TypeVar and Generic type hints. They're commonly used to make data structures type-safe while remaining reusable.

Iswanna added 8 commits June 30, 2026 12:56

feat: implement Node class for doubly linked list

5ee360b

- Define Node class to store value - Add pointers for next and previous nodes

feat: add LinkedList class and initialize head and tail pointers

c993128

- Define the LinkedList class in linked_list.py. - Implement the __init__ method to track the head and tail of the list. - Initialize head and tail to None for an empty list state.

feat: implement push_head method in LinkedList

47ed015

- Added the logic to create a new node and insert it at the front of the list. - Handled pointer updates for both empty and non-empty list states. - Returns the new node to allow for future removal/referencing.

Implement pop_tail method in LinkedList

a15f352

- Added logic to remove the last node from the list. - Handles edge cases for empty lists and single-node lists. - Updates tail pointers and returns the value of the removed node.

feat: implement remove method in LinkedList

eaaa1f9

- Added logic to remove a specific node from any position. - Included pointer updates for head and tail edge cases. - Added neighbor re-linking to maintain list integrity.

test: add unit tests for node removal and edge cases

dd21292

- Added test_remove_middle to verify pointer re-linking when deleting from the center. - Added test_pop_empty_list to ensure the program doesn't crash on empty lists. - Added test_remove_none to verify handling of null input.

docs: add CHANGES-MADE.md documenting Linked List implementation

8935eb0

- Documented core logic for Node and LinkedList classes - Explained testing strategy for edge cases (empty lists, middle removal) - Traded extra memory space to achieve O(1) time (constant speed).

docs: update terminology in CHANGES-MADE.md

2f0b1b7

- Changed "Linked List" to "Doubly Linked List" to accurately reflect the bi-directional implementation. - Updated header for better technical clarity.

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Iswanna added 📅 Sprint 2 Assigned during Sprint 2 of this module Needs Review Trainee to add when requesting review. PRs without this label will not be reviewed. Module-Complexity The name of the module. labels Jun 30, 2026

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Iswanna added the Needs Review Trainee to add when requesting review. PRs without this label will not be reviewed. label Jul 2, 2026

cjyuan reviewed Jul 2, 2026

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cjyuan added Reviewed Volunteer to add when completing a review with trainee action still to take. and removed Needs Review Trainee to add when requesting review. PRs without this label will not be reviewed. labels Jul 2, 2026

Iswanna added 4 commits July 3, 2026 11:50

perf: implement __slots__ in Node class to optimize memory

b756034

- Add __slots__ to prevent per-instance dictionary overhead - Optimize space complexity for large-scale list usage

refactor: add type hints to Node and LinkedList classes

6e4d93a

- Implement typing (Any, Optional) for parameters and return values - Add forward references for Node pointers

refactor: simplify pop_tail by reusing remove method

1e507e2

- Replace manual pointer logic in pop_tail with a call to self.remove() - Adhere to DRY (Don't Repeat Yourself) principles - Reduce code duplication and improve maintainability

Iswanna added Needs Review Trainee to add when requesting review. PRs without this label will not be reviewed. and removed Reviewed Volunteer to add when completing a review with trainee action still to take. labels Jul 3, 2026

Iswanna requested a review from cjyuan July 3, 2026 11:26

cjyuan reviewed Jul 3, 2026

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cjyuan added Complete Volunteer to add when work is complete and all review comments have been addressed. and removed Needs Review Trainee to add when requesting review. PRs without this label will not be reviewed. labels Jul 3, 2026

		# Logic 1: Store the value to return later
		value_to_return = self.tail.value

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Iswanna commented Jun 30, 2026 • edited Loading Uh oh! There was an error while loading. Please reload this page.

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Learners, PR Template

Changelist

Key Changes

Testing Done

Learning Reflection (Trade-offs)

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cjyuan Jul 2, 2026 • edited Loading Uh oh! There was an error while loading. Please reload this page.

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cjyuan Jul 2, 2026 • edited Loading Uh oh! There was an error while loading. Please reload this page.

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Iswanna commented Jun 30, 2026 •

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