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"""Exceptions used by the library."""


class PygraphError(Exception):
    """Root exception class for all library exceptions. Only used for subclassing."""
    pass


class NonexistentNodeError(PygraphError):
    """Thrown when a node does not exist within a graph."""
    def __init__(self, node_id):
        self.node_id = node_id

    def __str__(self):
        return 'Node "{}" does not exist.'.format(self.node_id)


class NonexistentEdgeError(PygraphError):
    """Thrown when an edge does not exist within a graph."""
    def __init__(self, edge_id):
        self.edge_id = edge_id

    def __str__(self):
        return 'Edge "{}" does not exist.'.format(self.edge_id)


class DisconnectedGraphError(PygraphError):
    """Thrown when a graph is disconnected (and such is unexpected by an algorithm)."""
    pass