cpython/Lib/graphlib.py at 3.15 · python/cpython · GitHub

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from types import GenericAlias

__all__ = ["TopologicalSorter", "CycleError"]

_NODE_OUT = -1

_NODE_DONE = -2

class _NodeInfo:

__slots__ = "node", "npredecessors", "successors"

def __init__(self, node):

# The node this class is augmenting.

self.node = node

# Number of predecessors, generally >= 0. When this value falls to 0,

# and is returned by get_ready(), this is set to _NODE_OUT and when the

# node is marked done by a call to done(), set to _NODE_DONE.

self.npredecessors = 0

# List of successor nodes. The list can contain duplicated elements as

# long as they're all reflected in the successor's npredecessors attribute.

self.successors = []

class CycleError(ValueError):

"""Subclass of ValueError raised by 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 *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.

"""

pass

class TopologicalSorter:

"""Provides functionality to topologically sort a graph of hashable nodes"""

def __init__(self, graph=None):

self._node2info = {}

self._ready_nodes = None

self._npassedout = 0

self._nfinished = 0

if graph is not None:

for node, predecessors in graph.items():

self.add(node, *predecessors)

def _get_nodeinfo(self, node):

if (result := self._node2info.get(node)) is None:

self._node2info[node] = result = _NodeInfo(node)

return result

def add(self, 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)

as well as 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 ValueError if called after "prepare".

"""

if self._ready_nodes is not None:

raise ValueError("Nodes cannot be added after a call to prepare()")

# Create the node -> predecessor edges

nodeinfo = self._get_nodeinfo(node)

nodeinfo.npredecessors += len(predecessors)

# Create the predecessor -> node edges

for pred in predecessors:

pred_info = self._get_nodeinfo(pred)

pred_info.successors.append(node)

def prepare(self):

"""Mark the graph as finished and check for cycles in the graph.

If any cycle is detected, "CycleError" will be raised, but "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 "add".

Raise ValueError if nodes have already been passed out of the sorter.

"""

if self._npassedout > 0:

raise ValueError("cannot prepare() after starting sort")

if self._ready_nodes is None:

self._ready_nodes = [

i.node for i in self._node2info.values() if i.npredecessors == 0

]

# ready_nodes is set before we look for cycles on purpose:

# if the user wants to catch the CycleError, that's fine,

# they can continue using the instance to grab as many

# nodes as possible before cycles block more progress

cycle = self._find_cycle()

if cycle:

raise CycleError("nodes are in a cycle", cycle)

def get_ready(self):

"""Return a tuple of all the nodes that are ready.

Initially it returns all nodes with no predecessors; once those are marked

as processed by calling "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 ValueError if called without calling "prepare" previously.

"""

if self._ready_nodes is None:

raise ValueError("prepare() must be called first")

# Get the nodes that are ready and mark them

result = tuple(self._ready_nodes)

n2i = self._node2info

for node in result:

n2i[node].npredecessors = _NODE_OUT

# Clean the list of nodes that are ready and update

# the counter of nodes that we have returned.

self._ready_nodes.clear()

self._npassedout += len(result)

return result

def is_active(self):

"""Return ``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 "get_ready" or the

number of nodes marked "done" is less than the number that have been returned

by "get_ready".

Raises ValueError if called without calling "prepare" previously.

"""

if self._ready_nodes is None:

raise ValueError("prepare() must be called first")

return self._nfinished < self._npassedout or bool(self._ready_nodes)

def __bool__(self):

return self.is_active()

def done(self, *nodes):

"""Marks a set of nodes returned by "get_ready" as processed.

This method unblocks any successor of each node in *nodes* for being returned

in the future by a call to "get_ready".

Raises ValueError if any node in *nodes* has already been marked as

processed by a previous call to this method, if a node was not added to the

graph by using "add" or if called without calling "prepare" previously or if

node has not yet been returned by "get_ready".

"""

if self._ready_nodes is None:

raise ValueError("prepare() must be called first")

n2i = self._node2info

for node in nodes:

# Check if we know about this node (it was added previously using add()

if (nodeinfo := n2i.get(node)) is None:

raise ValueError(f"node {node!r} was not added using add()")

# If the node has not being returned (marked as ready) previously, inform the user.

stat = nodeinfo.npredecessors

if stat != _NODE_OUT:

if stat >= 0:

raise ValueError(

f"node {node!r} was not passed out (still not ready)"

)

elif stat == _NODE_DONE:

raise ValueError(f"node {node!r} was already marked done")

else:

assert False, f"node {node!r}: unknown status {stat}"

# Mark the node as processed

nodeinfo.npredecessors = _NODE_DONE

# Go to all the successors and reduce the number of predecessors, collecting all the ones

# that are ready to be returned in the next get_ready() call.

for successor in nodeinfo.successors:

successor_info = n2i[successor]

successor_info.npredecessors -= 1

if successor_info.npredecessors == 0:

self._ready_nodes.append(successor)

self._nfinished += 1

# See note "On Finding Cycles" at the bottom.

def _find_cycle(self):

n2i = self._node2info

stack = []

itstack = []

seen = set()

node2stacki = {}

for node in n2i:

if node in seen:

continue

while True:

if node in seen:

if node in node2stacki:

return stack[node2stacki[node] :] + [node]

# else go on to get next successor

else:

seen.add(node)

itstack.append(iter(n2i[node].successors).__next__)

node2stacki[node] = len(stack)

stack.append(node)

# Backtrack to the topmost stack entry with

# at least another successor.

while stack:

try:

node = itstack[-1]()

break # resume at top of "while True:"

except StopIteration:

# no more successors; pop the stack

# and continue looking up

del node2stacki[stack.pop()]

itstack.pop()

else:

# stack is empty; look for a fresh node to

# start over from (a node not yet in seen)

break

return None

def static_order(self):

"""Returns an iterable of nodes in a topological order.

The particular order that is returned may depend on the specific

order in which the items were inserted in the graph.

Using this method does not require to call "prepare" or "done". If any

cycle is detected, :exc:`CycleError` will be raised.

"""

self.prepare()

while self.is_active():

node_group = self.get_ready()

yield from node_group

self.done(*node_group)

__class_getitem__ = classmethod(GenericAlias)

# On Finding Cycles

# -----------------

# There is a (at least one) total order if and only if the graph is

# acyclic.

# When it is cyclic, "there's a cycle - somewhere!" isn't very helpful.

# In theory, it would be most helpful to partition the graph into

# strongly connected components (SCCs) and display those with more than

# one node. Then all cycles could easily be identified "by eyeball".

# That's a lot of work, though, and we can get most of the benefit much

# more easily just by showing a single specific cycle.

# Approaches to that are based on breadth first or depth first search

# (BFS or DFS). BFS is most natural, which can easily be arranged to

# find a shortest-possible cycle. But memory burden can be high, because

# every path-in-progress has to keep its own idea of what "the path" is

# so far.

# DFS is much easier on RAM, only requiring keeping track of _the_ path

# from the starting node to the current node at the current recursion

# level. But there may be any number of nodes, and so there's no bound

# on recursion depth short of the total number of nodes.

# So we use an iterative version of DFS, keeping an exploit list

# (`stack`) of the path so far. A parallel stack (`itstack`) holds the

# `__next__` method of an iterator over the current level's node's

# successors, so when backtracking to a shallower level we can just call

# that to get the node's next successor. This is state that a recursive

# version would implicitly store in a `for` loop's internals.

# `seen()` is a set recording which nodes have already been, at some

# time, pushed on the stack. If a node has been pushed on the stack, DFS

# will find any cycle it's part of, so there's no need to ever look at

# it again.

# Finally, `node2stacki` maps a node to its index on the current stack,

# for and only for nodes currently _on_ the stack. If a successor to be

# pushed on the stack is in that dict, the node is already on the path,

# at that index. The cycle is then `stack[that_index :] + [node]`.

# As is often the case when removing recursion, the control flow looks a

# bit off. The "while True:" loop here rarely actually loops - it's only

# looking to go "up the stack" until finding a level that has another

# successor to consider, emulating a chain of returns in a recursive

# version.

# Worst cases: O(V+E) for time, and O(V) for memory, where V is the

# number of nodes and E the number of edges (which may be quadratic in

# V!). It requires care to ensure these bounds are met.

cpython/Lib/graphlib.py at 3.15 · python/cpython · GitHub

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