AtsushiSakai/PythonRobotics · error · Exception

Agent index cannot be 0

Error message

Agent index cannot be 0

What it means

Raised by reserve_path when agent_index is 0. Index 0 is the sentinel stored in reservation_matrix to mean 'unreserved', so it cannot be used as a real agent identifier. Agents are expected to be numbered starting from 1.

Source

Thrown at PathPlanning/TimeBasedPathPlanning/GridWithDynamicObstacles.py:299

        if zero_mask[-1]:  # If the last element is zero, add the last index to end_indices
            end_indices = np.append(end_indices, len(vals) - 1)

        # Create pairs of (first zero, last zero)
        intervals = [Interval(int(start), int(end)) for start, end in zip(start_indices, end_indices)]

        # Remove intervals where a cell is only free for one time step. Those intervals not provide enough time to
        # move into and out of the cell each take 1 time step, and the cell is considered occupied during
        # both the time step when it is entering the cell,  and the time step when it is leaving the cell.
        intervals = [interval for interval in intervals if interval.start_time != interval.end_time]
        return intervals
    
    """
    Reserve an agent's path in the grid. Raises an exception if the agent's index is 0, or if a position is
    already reserved by a different agent.
    """
    def reserve_path(self, node_path: NodePath, agent_index: int):
        if agent_index == 0:
            raise Exception("Agent index cannot be 0")
        
        for i, node in enumerate(node_path.path):
            reservation_finish_time = node.time + 1
            if i < len(node_path.path) - 1:
                reservation_finish_time = node_path.path[i + 1].time

            self.reserve_position(node.position, agent_index, Interval(node.time, reservation_finish_time))

    """
    Reserve a position for the provided agent during the provided time interval.
    Raises an exception if the agent's index is 0, or if the position is already reserved by a different agent during the interval.
    """
    def reserve_position(self, position: Position, agent_index: int, interval: Interval):
        if agent_index == 0:
            raise Exception("Agent index cannot be 0")

        for t in range(interval.start_time, interval.end_time + 1):
            current_reserver = self.reservation_matrix[position.x, position.y, t]

View on GitHub (pinned to 1fe4fb980f)

Solutions

1. Change the call to use agent_index >= 1 (e.g. use i + 1 in a 0-based loop)
2. Audit agent ID assignment so agents are numbered 1..N throughout the codebase

Example fix

# before
for i in range(len(paths)):
    grid.reserve_path(paths[i], i)

# after
for i in range(len(paths)):
    grid.reserve_path(paths[i], i + 1)

assert agent_index >= 1, "agent_index must be >= 1 (0 is the unreserved sentinel)"

def is_valid_agent_index(idx: int) -> bool:
    return isinstance(idx, int) and idx >= 1

When it happens

Trigger: Calling grid.reserve_path(node_path, agent_index=0) directly, or via plan(...) passing an agent index of 0 (e.g. iterating agents from a 0-based loop without adding 1).

Common situations: Using a 0-based loop `for i in range(num_agents): plan(path, i)` instead of 1-based indexing; off-by-one errors after refactoring agent IDs; passing a default value of 0 for agent_index.

Related errors

• Agent {agent_index} tried to reserve a position already rese
• Number of obstacles is greater than grid size!
• Path position not found for time {i}.
• No path found
• No path found

AI-assisted analysis of AtsushiSakai/PythonRobotics@1fe4fb980f (2026-08-28). Data as JSON: /api/errors/f32f14fa39e823f8. Report an issue: GitHub.