bug: add_activities_relations() silently accepts duplicate activity names — adding an activity "A" twice overwrites the first node without warning, producing incorrect CPM calculations with no error raised

[prince-pokharna]

🐛 Problem Statement

The CriticalPathMethod.add_activities_relations() method accepts a list of activity names and adds them to the internal node dictionary. If a caller passes the same activity name twice — either by mistake or by calling the method in a loop — the second call silently overwrites the first node:

cpm = CriticalPathMethod()
cpm.add_activity('O', 0)
cpm.add_activities_relations(['A', 'B', 'A'], [2, 5, 99], ['-', 'A', 'B'])
# Activity 'A' is now duration=99 with predecessor 'B' — wrong!

This produces:

• Incorrect Early Start / Early Finish times for the overwritten node
• A wrong Critical Path (the overwritten node may become critical with the new duration)
• No ValueError or warning — the user receives silently wrong output

For a library used in project management / academic contexts where CPM accuracy is critical, this is a data integrity bug.

💡 Proposed Fix

Add duplicate detection in add_activities_relations() and add_activity():

# networkdiagram/cpm.py

def add_activity(self, name: str, duration: float) -> None:
    """
    Add a single activity node to the network.

    Raises:
        ValueError: If an activity with this name already exists.
    """
    if name in self.nodes:
        raise ValueError(
            f"Activity '{name}' already exists in the network. "
            f"Duplicate activity names are not allowed. "
            f"Existing node: duration={self.nodes[name].duration}"
        )
    self.nodes[name] = Node(name=name, duration=duration)

def add_activities_relations(
    self,
    activities: list[str],
    durations: list[float],
    predecessors: list[str]
) -> None:
    """
    Add multiple activities with their durations and predecessor relationships.

    Raises:
        ValueError: If activity lists have mismatched lengths.
        ValueError: If any activity name is duplicated within the input list.
        ValueError: If any activity name already exists in the network.
    """
    if not (len(activities) == len(durations) == len(predecessors)):
        raise ValueError(
            f"Length mismatch: activities={len(activities)}, "
            f"durations={len(durations)}, predecessors={len(predecessors)}. "
            "All three lists must have the same length."
        )

    # Check for duplicates within the input list itself
    seen = set()
    for name in activities:
        if name in seen:
            raise ValueError(
                f"Duplicate activity name '{name}' found in the input list. "
                "Each activity must have a unique name."
            )
        seen.add(name)

    # Add all activities (add_activity checks for existing network conflicts)
    for name, duration in zip(activities, durations):
        self.add_activity(name, duration)

    # Then set up predecessor relationships
    for name, pred_str in zip(activities, predecessors):
        self.nodes[name].predecessors = self._parse_predecessors(pred_str)

Add unit tests covering the duplicate case:

# tests/test_duplicate_activities.py

import pytest
from networkdiagram import CriticalPathMethod

def test_duplicate_in_input_list_raises():
    cpm = CriticalPathMethod()
    cpm.add_activity('O', 0)
    with pytest.raises(ValueError, match="Duplicate activity name 'A'"):
        cpm.add_activities_relations(['A', 'B', 'A'], [2, 5, 3], ['-', 'A', 'B'])

def test_add_existing_activity_raises():
    cpm = CriticalPathMethod()
    cpm.add_activity('O', 0)
    cpm.add_activity('A', 5)
    with pytest.raises(ValueError, match="Activity 'A' already exists"):
        cpm.add_activity('A', 10)

def test_unique_activities_no_error():
    cpm = CriticalPathMethod()
    cpm.add_activity('O', 0)
    cpm.add_activities_relations(['A', 'B', 'C'], [2, 5, 3], ['-', 'A', 'A'])
    assert 'A' in cpm.nodes
    assert 'C' in cpm.nodes

📁 Files to Modify

File	Change
networkdiagram/cpm.py	Add duplicate check in add_activity() and add_activities_relations()
tests/test_duplicate_activities.py	New — unit tests for duplicate detection

✅ Acceptance Criteria

• add_activity('A', 5) followed by add_activity('A', 10) raises ValueError
• add_activities_relations(['A', 'B', 'A'], ...) raises ValueError with the duplicate name
• All existing tests continue to pass
• Error messages are descriptive and include the duplicate name

Suggested labels: bug, data-integrity, validation

I would like to work on this. Could you please assign it to me?

[prince-pokharna]

Hey @kathan-majithia ,
Please assign it to me under GSSoC 2026.
/claim /assign /claim-assignment