bpo-17005: Move topological sort functionality to its own module (GH-20558)

The topological sort functionality that was introduced initially in the
functools module has been moved to a new graphlib module to
better accommodate the new tools and keep the original scope of the
functools module.

Author: pablogsal

Doc/library/datatypes.rst

@@ -33,3 +33,4 @@ The following modules are documented in this chapter:
    pprint.rst
    reprlib.rst
    enum.rst
+   graphlib.rst

Doc/library/functools.rst

@@ -543,184 +543,6 @@ The :mod:`functools` module defines the following functions:
    .. versionadded:: 3.8
 
 
-.. class:: TopologicalSorter(graph=None)
-
-   Provides functionality to topologically sort a graph of hashable nodes.
-
-   A topological order is a linear ordering of the vertices in a graph such that
-   for every directed edge u -> v from vertex u to vertex v, vertex u comes
-   before vertex v in the ordering. For instance, the vertices of the graph may
-   represent tasks to be performed, and the edges may represent constraints that
-   one task must be performed before another; in this example, a topological
-   ordering is just a valid sequence for the tasks. A complete topological
-   ordering is possible if and only if the graph has no directed cycles, that
-   is, if it is a directed acyclic graph.
-
-   If the optional *graph* argument is provided it must be a dictionary
-   representing a directed acyclic graph where the keys are nodes and the values
-   are iterables of all predecessors of that node in the graph (the nodes that
-   have edges that point to the value in the key). Additional nodes can be added
-   to the graph using the :meth:`~TopologicalSorter.add` method.
-
-   In the general case, the steps required to perform the sorting of a given
-   graph are as follows:
-
-         * Create an instance of the :class:`TopologicalSorter` with an optional
-           initial graph.
-         * Add additional nodes to the graph.
-         * Call :meth:`~TopologicalSorter.prepare` on the graph.
-         * While :meth:`~TopologicalSorter.is_active` is ``True``, iterate over
-           the nodes returned by :meth:`~TopologicalSorter.get_ready` and
-           process them. Call :meth:`~TopologicalSorter.done` on each node as it
-           finishes processing.
-
-   In case just an immediate sorting of the nodes in the graph is required and
-   no parallelism is involved, the convenience method
-   :meth:`TopologicalSorter.static_order` can be used directly:
-
-   .. doctest::
-
-       >>> graph = {"D": {"B", "C"}, "C": {"A"}, "B": {"A"}}
-       >>> ts = TopologicalSorter(graph)
-       >>> tuple(ts.static_order())
-       ('A', 'C', 'B', 'D')
-
-   The class is designed to easily support parallel processing of the nodes as
-   they become ready. For instance::
-
-       topological_sorter = TopologicalSorter()
-
-       # Add nodes to 'topological_sorter'...
-
-       topological_sorter.prepare()
-       while topological_sorter.is_active():
-           for node in topological_sorter.get_ready():
-               # Worker threads or processes take nodes to work on off the
-               # 'task_queue' queue.
-               task_queue.put(node)
-
-           # When the work for a node is done, workers put the node in
-           # 'finalized_tasks_queue' so we can get more nodes to work on.
-           # The definition of 'is_active()' guarantees that, at this point, at
-           # least one node has been placed on 'task_queue' that hasn't yet
-           # been passed to 'done()', so this blocking 'get()' must (eventually)
-           # succeed.  After calling 'done()', we loop back to call 'get_ready()'
-           # again, so put newly freed nodes on 'task_queue' as soon as
-           # logically possible.
-           node = finalized_tasks_queue.get()
-           topological_sorter.done(node)
-
-   .. method:: add(node, *predecessors)
-
-      Add a new node and its predecessors to the graph. Both the *node* and all
-      elements in *predecessors* must be hashable.
-
-      If called multiple times with the same node argument, the set of
-      dependencies will be the union of all dependencies passed in.
-
-      It is possible to add a node with no dependencies (*predecessors* is not
-      provided) or to provide a dependency twice. If a node that has not been
-      provided before is included among *predecessors* it will be automatically
-      added to the graph with no predecessors of its own.
-
-      Raises :exc:`ValueError` if called after :meth:`~TopologicalSorter.prepare`.
-
-   .. method:: prepare()
-
-      Mark the graph as finished and check for cycles in the graph. If any cycle
-      is detected, :exc:`CycleError` will be raised, but
-      :meth:`~TopologicalSorter.get_ready` can still be used to obtain as many
-      nodes as possible until cycles block more progress. After a call to this
-      function, the graph cannot be modified, and therefore no more nodes can be
-      added using :meth:`~TopologicalSorter.add`.
-
-   .. method:: is_active()
-
-      Returns ``True`` if more progress can be made and ``False`` otherwise.
-      Progress can be made if cycles do not block the resolution and either
-      there are still nodes ready that haven't yet been returned by
-      :meth:`TopologicalSorter.get_ready` or the number of nodes marked
-      :meth:`TopologicalSorter.done` is less than the number that have been
-      returned by :meth:`TopologicalSorter.get_ready`.
-
-      The :meth:`~TopologicalSorter.__bool__` method of this class defers to
-      this function, so instead of::
-
-          if ts.is_active():
-              ...
-
-      if possible to simply do::
-
-          if ts:
-              ...
-
-      Raises :exc:`ValueError` if called without calling
-      :meth:`~TopologicalSorter.prepare` previously.
-
-   .. method:: done(*nodes)
-
-      Marks a set of nodes returned by :meth:`TopologicalSorter.get_ready` as
-      processed, unblocking any successor of each node in *nodes* for being
-      returned in the future by a call to :meth:`TopologicalSorter.get_ready`.
-
-      Raises :exc:`ValueError` if any node in *nodes* has already been marked as
-      processed by a previous call to this method or if a node was not added to
-      the graph by using :meth:`TopologicalSorter.add`, if called without
-      calling :meth:`~TopologicalSorter.prepare` or if node has not yet been
-      returned by :meth:`~TopologicalSorter.get_ready`.
-
-   .. method:: get_ready()
-
-      Returns a ``tuple`` with all the nodes that are ready. Initially it
-      returns all nodes with no predecessors, and once those are marked as
-      processed by calling :meth:`TopologicalSorter.done`, further calls will
-      return all new nodes that have all their predecessors already processed.
-      Once no more progress can be made, empty tuples are returned.
-
-      Raises :exc:`ValueError` if called without calling
-      :meth:`~TopologicalSorter.prepare` previously.
-
-   .. method:: static_order()
-
-      Returns an iterable of nodes in a topological order. Using this method
-      does not require to call :meth:`TopologicalSorter.prepare` or
-      :meth:`TopologicalSorter.done`. This method is equivalent to::
-
-          def static_order(self):
-              self.prepare()
-              while self.is_active():
-                  node_group = self.get_ready()
-                  yield from node_group
-                  self.done(*node_group)
-
-      The particular order that is returned may depend on the specific order in
-      which the items were inserted in the graph. For example:
-
-      .. doctest::
-
-          >>> ts = TopologicalSorter()
-          >>> ts.add(3, 2, 1)
-          >>> ts.add(1, 0)
-          >>> print([*ts.static_order()])
-          [2, 0, 1, 3]
-
-          >>> ts2 = TopologicalSorter()
-          >>> ts2.add(1, 0)
-          >>> ts2.add(3, 2, 1)
-          >>> print([*ts2.static_order()])
-          [0, 2, 1, 3]
-
-      This is due to the fact that "0" and "2" are in the same level in the
-      graph (they would have been returned in the same call to
-      :meth:`~TopologicalSorter.get_ready`) and the order between them is
-      determined by the order of insertion.
-
-
-      If any cycle is detected, :exc:`CycleError` will be raised.
-
-   .. versionadded:: 3.9
-
-
 .. function:: update_wrapper(wrapper, wrapped, assigned=WRAPPER_ASSIGNMENTS, updated=WRAPPER_UPDATES)
 
    Update a *wrapper* function to look like the *wrapped* function. The optional
@@ -829,20 +651,4 @@ callable, weak referencable, and can have attributes.  There are some important
 differences.  For instance, the :attr:`~definition.__name__` and :attr:`__doc__` attributes
 are not created automatically.  Also, :class:`partial` objects defined in
 classes behave like static methods and do not transform into bound methods
-during instance attribute look-up.
-
-
-Exceptions
-----------
-The :mod:`functools` module defines the following exception classes:
-
-.. exception:: CycleError
-
-   Subclass of :exc:`ValueError` raised by :meth:`TopologicalSorter.prepare` if cycles exist
-   in the working graph. If multiple cycles exist, only one undefined choice among them will
-   be reported and included in the exception.
-
-   The detected cycle can be accessed via the second element in the :attr:`~CycleError.args`
-   attribute of the exception instance and consists in a list of nodes, such that each node is,
-   in the graph, an immediate predecessor of the next node in the list. In the reported list,
-   the first and the last node will be the same, to make it clear that it is cyclic.
+during instance attribute look-up.