Refactor DAG internals and lazy-load networkx

CHANGELOG.md

@@ -7,6 +7,10 @@ releases are available on [PyPI](https://pypi.org/project/pytask) and
 
 ## Unreleased
 
+- [#830](https://github.com/pytask-dev/pytask/pull/830) replaces the internal
+  `networkx` dependency with a pytask-owned DAG implementation, lazy-loads
+  `networkx` only for DAG export and visualization, and makes the `networkx`
+  dependency optional for core builds.
 - [#822](https://github.com/pytask-dev/pytask/pull/822) fixes unstable signatures
   for remote `UPath`-backed `PathNode`s and `PickleNode`s so unchanged remote inputs
   are no longer reported as missing from the state database on subsequent runs.

docs/source/tutorials/visualizing_the_dag.md

@@ -1,12 +1,22 @@
 # Visualizing the DAG
 
 To visualize the [DAG](../glossary.md#dag) of the project, first, install
-[pygraphviz](https://github.com/pygraphviz/pygraphviz) and
-[graphviz](https://graphviz.org/). For example, you can both install with pixi
+[networkx](https://networkx.org/),
+[pygraphviz](https://github.com/pygraphviz/pygraphviz), and
+[graphviz](https://graphviz.org/).
 
-```console
-$ pixi add pygraphviz graphviz
-```
+=== "uv"
+
+    ```console
+    $ uv add networkx
+    $ uv add --optional dag pygraphviz
+    ```
+
+=== "pixi"
+
+    ```console
+    $ pixi add networkx pygraphviz graphviz
+    ```
 
 After that, pytask offers two interfaces to visualize your project's `DAG`.
 

pyproject.toml

@@ -24,7 +24,6 @@ dependencies = [
     "click>=8.1.8,!=8.2.0",
     "click-default-group>=1.2.4",
     "msgspec>=0.18.6",
-    "networkx>=2.4.0",
     "optree>=0.9.0",
     "packaging>=23.0.0",
     "pluggy>=1.3.0",
@@ -36,6 +35,9 @@ dependencies = [
     "universal-pathlib>=0.2.2",
 ]
 
+[project.optional-dependencies]
+dag = ["networkx>=2.4.0"]
+
 [project.readme]
 file = "README.md"
 content-type = "text/markdown"
@@ -54,6 +56,7 @@ docs = [
     "ipywidgets>=8.1.6",
     "matplotlib>=3.5.0",
     "mkdocstrings[python]>=0.30.0",
+    "networkx>=2.4.0",
     "zensical>=0.0.23",
 ]
 docs-live = ["sphinx-autobuild>=2024.10.3"]
@@ -71,6 +74,7 @@ test = [
     "syrupy>=4.5.0",
     "aiohttp>=3.11.0",                                                        # For HTTPPath tests.
     "coiled>=1.42.0; python_version < '3.14'",
+    "networkx>=2.4.0",
     "pygraphviz>=1.12;platform_system=='Linux'",
 ]
 typing = ["ty>=0.0.8"]

src/_pytask/dag.py

@@ -6,7 +6,6 @@
 import sys
 from typing import TYPE_CHECKING
 
-import networkx as nx
 from rich.text import Text
 from rich.tree import Tree
 
@@ -17,6 +16,9 @@
 from _pytask.console import format_node_name
 from _pytask.console import format_task_name
 from _pytask.console import render_to_string
+from _pytask.dag_graph import DAG
+from _pytask.dag_graph import NoCycleError
+from _pytask.dag_graph import find_cycle
 from _pytask.exceptions import ResolvingDependenciesError
 from _pytask.mark import select_by_after_keyword
 from _pytask.mark import select_tasks_by_marks_and_expressions
@@ -37,7 +39,7 @@
 __all__ = ["create_dag", "create_dag_from_session"]
 
 
-def create_dag(session: Session) -> nx.DiGraph:
+def create_dag(session: Session) -> DAG:
     """Create a directed acyclic graph (DAG) for the workflow."""
     try:
         dag = create_dag_from_session(session)
@@ -50,7 +52,7 @@ def create_dag(session: Session) -> nx.DiGraph:
     return dag
 
 
-def create_dag_from_session(session: Session) -> nx.DiGraph:
+def create_dag_from_session(session: Session) -> DAG:
     """Create a DAG from a session."""
     dag = _create_dag_from_tasks(tasks=session.tasks)
     _check_if_dag_has_cycles(dag)
@@ -60,14 +62,16 @@ def create_dag_from_session(session: Session) -> nx.DiGraph:
     return dag
 
 
-def _create_dag_from_tasks(tasks: list[PTask]) -> nx.DiGraph:
+def _create_dag_from_tasks(tasks: list[PTask]) -> DAG:
     """Create the DAG from tasks, dependencies and products."""
 
-    def _add_dependency(
-        dag: nx.DiGraph, task: PTask, node: PNode | PProvisionalNode
-    ) -> None:
+    def _add_node_data(dag: DAG, node: PNode | PProvisionalNode) -> None:
+        dag.add_node(node.signature, node)
+        if isinstance(node, PythonNode) and isinstance(node.value, PythonNode):
+            _add_node_data(dag, node.value)
+
+    def _add_dependency(dag: DAG, task: PTask, node: PNode | PProvisionalNode) -> None:
         """Add a dependency to the DAG."""
-        dag.add_node(node.signature, node=node)
         dag.add_edge(node.signature, task.signature)
 
         # If a node is a PythonNode wrapped in another PythonNode, it is a product from
@@ -76,36 +80,24 @@ def _add_dependency(
         if isinstance(node, PythonNode) and isinstance(node.value, PythonNode):
             dag.add_edge(node.value.signature, node.signature)
 
-    def _add_product(
-        dag: nx.DiGraph, task: PTask, node: PNode | PProvisionalNode
-    ) -> None:
+    def _add_product(dag: DAG, task: PTask, node: PNode | PProvisionalNode) -> None:
         """Add a product to the DAG."""
-        dag.add_node(node.signature, node=node)
         dag.add_edge(task.signature, node.signature)
 
-    dag = nx.DiGraph()
+    dag = DAG()
 
     for task in tasks:
-        dag.add_node(task.signature, task=task)
+        dag.add_node(task.signature, task)
+        tree_map(lambda x: _add_node_data(dag, x), task.depends_on)
+        tree_map(lambda x: _add_node_data(dag, x), task.produces)
 
+    for task in tasks:
         tree_map(lambda x: _add_dependency(dag, task, x), task.depends_on)
         tree_map(lambda x: _add_product(dag, task, x), task.produces)
-
-        # If a node is a PythonNode wrapped in another PythonNode, it is a product from
-        # another task that is a dependency in the current task. Thus, draw an edge
-        # connecting the two nodes.
-        tree_map(
-            lambda x: (
-                dag.add_edge(x.value.signature, x.signature)
-                if isinstance(x, PythonNode) and isinstance(x.value, PythonNode)
-                else None
-            ),
-            task.depends_on,
-        )
     return dag
 
 
-def _modify_dag(session: Session, dag: nx.DiGraph) -> nx.DiGraph:
+def _modify_dag(session: Session, dag: DAG) -> DAG:
     """Create dependencies between tasks when using ``@task(after=...)``."""
     temporary_id_to_task = {
         task.attributes["collection_id"]: task
@@ -129,11 +121,11 @@ def _modify_dag(session: Session, dag: nx.DiGraph) -> nx.DiGraph:
     return dag
 
 
-def _check_if_dag_has_cycles(dag: nx.DiGraph) -> None:
+def _check_if_dag_has_cycles(dag: DAG) -> None:
     """Check if DAG has cycles."""
     try:
-        cycles = nx.algorithms.cycles.find_cycle(dag)
-    except nx.NetworkXNoCycle:
+        cycles = find_cycle(dag)
+    except NoCycleError:
         pass
     else:
         msg = (
@@ -145,7 +137,7 @@ def _check_if_dag_has_cycles(dag: nx.DiGraph) -> None:
         raise ResolvingDependenciesError(msg)
 
 
-def _format_cycles(dag: nx.DiGraph, cycles: list[tuple[str, ...]]) -> str:
+def _format_cycles(dag: DAG, cycles: list[tuple[str, str]]) -> str:
     """Format cycles as a paths connected by arrows."""
     chain = [
         x for i, x in enumerate(itertools.chain.from_iterable(cycles)) if i % 2 == 0
@@ -154,7 +146,7 @@ def _format_cycles(dag: nx.DiGraph, cycles: list[tuple[str, ...]]) -> str:
 
     lines: list[str] = []
     for x in chain:
-        node = dag.nodes[x].get("task") or dag.nodes[x].get("node")
+        node = dag.nodes[x]
         if isinstance(node, PTask):
             short_name = format_task_name(node, editor_url_scheme="no_link").plain
         elif isinstance(node, (PNode, PProvisionalNode)):
@@ -176,24 +168,27 @@ def _format_dictionary_to_tree(dict_: dict[str, list[str]], title: str) -> str:
     return render_to_string(tree, console=console, strip_styles=True)
 
 
-def _check_if_tasks_have_the_same_products(dag: nx.DiGraph, paths: list[Path]) -> None:
+def _check_if_tasks_have_the_same_products(dag: DAG, paths: list[Path]) -> None:
     nodes_created_by_multiple_tasks = []
 
     for node in dag.nodes:
-        is_node = "node" in dag.nodes[node]
-        if is_node:
+        if isinstance(dag.nodes[node], (PNode, PProvisionalNode)):
             parents = list(dag.predecessors(node))
             if len(parents) > 1:
                 nodes_created_by_multiple_tasks.append(node)
 
     if nodes_created_by_multiple_tasks:
         dictionary = {}
         for node in nodes_created_by_multiple_tasks:
-            short_node_name = format_node_name(dag.nodes[node]["node"], paths).plain
+            payload = dag.nodes[node]
+            if not isinstance(payload, (PNode, PProvisionalNode)):
+                msg = f"Expected product node for signature {node!r}."
+                raise TypeError(msg)
+            short_node_name = format_node_name(payload, paths).plain
             short_predecessors = reduce_names_of_multiple_nodes(
                 dag.predecessors(node), dag, paths
             )
-            dictionary[short_node_name] = short_predecessors
+            dictionary[short_node_name] = sorted(short_predecessors)
         text = _format_dictionary_to_tree(dictionary, "Products from multiple tasks:")
         msg = (
             f"There are some tasks which produce the same output. See the following "

src/_pytask/dag_command.py

@@ -5,11 +5,11 @@
 import enum
 import sys
 from pathlib import Path
+from typing import TYPE_CHECKING
 from typing import Any
 from typing import cast
 
 import click
-import networkx as nx
 from rich.text import Text
 
 from _pytask.click import ColoredCommand
@@ -30,6 +30,11 @@
 from _pytask.shared import reduce_names_of_multiple_nodes
 from _pytask.traceback import Traceback
 
+if TYPE_CHECKING:
+    import networkx as nx
+
+    from _pytask.dag_graph import DAG
+
 
 class _RankDirection(enum.Enum):
     TB = "TB"
@@ -92,6 +97,7 @@ def dag(**raw_config: Any) -> int:
     else:
         try:
             session.hook.pytask_log_session_header(session=session)
+            import_optional_dependency("networkx")
             import_optional_dependency("pygraphviz")
             check_for_optional_program(
                 session.config["layout"],
@@ -100,7 +106,7 @@ def dag(**raw_config: Any) -> int:
             )
             session.hook.pytask_collect(session=session)
             session.dag = create_dag(session=session)
-            dag = _refine_dag(session)
+            dag = _to_visualization_graph(session)
             _write_graph(dag, session.config["output_path"], session.config["layout"])
 
         except CollectionError:  # pragma: no cover
@@ -163,6 +169,7 @@ def build_dag(raw_config: dict[str, Any]) -> nx.DiGraph:
 
     else:
         session.hook.pytask_log_session_header(session=session)
+        import_optional_dependency("networkx")
         import_optional_dependency("pygraphviz")
         check_for_optional_program(
             session.config["layout"],
@@ -172,44 +179,42 @@ def build_dag(raw_config: dict[str, Any]) -> nx.DiGraph:
         session.hook.pytask_collect(session=session)
         session.dag = create_dag(session=session)
         session.hook.pytask_unconfigure(session=session)
-        return _refine_dag(session)
+        return _to_visualization_graph(session)
 
 
-def _refine_dag(session: Session) -> nx.DiGraph:
+def _refine_dag(session: Session) -> DAG:
     """Refine the dag for plotting."""
     dag = _shorten_node_labels(session.dag, session.config["paths"])
-    dag = _clean_dag(dag)
-    dag = _style_dag(dag)
-    dag.graph["graph"] = {"rankdir": session.config["rank_direction"].name}
+    return _clean_dag(dag)
 
+
+def _to_visualization_graph(session: Session) -> nx.DiGraph:
+    """Convert the internal DAG to a styled networkx graph for visualization."""
+    nx = cast("Any", import_optional_dependency("networkx"))
+    dag = _refine_dag(session).to_networkx()
+    dag.graph["graph"] = {"rankdir": session.config["rank_direction"].name}
+    shapes = {name: "hexagon" if "::task_" in name else "box" for name in dag.nodes}
+    nx.set_node_attributes(dag, shapes, "shape")
     return dag
 
 
-def _shorten_node_labels(dag: nx.DiGraph, paths: list[Path]) -> nx.DiGraph:
+def _shorten_node_labels(dag: DAG, paths: list[Path]) -> DAG:
     """Shorten the node labels in the graph for a better experience."""
     node_names = dag.nodes
     short_names = reduce_names_of_multiple_nodes(node_names, dag, paths)
     short_names = [i.plain if isinstance(i, Text) else i for i in short_names]
     old_to_new = dict(zip(node_names, short_names, strict=False))
-    return nx.relabel_nodes(dag, old_to_new)
+    return dag.relabel_nodes(old_to_new)
 
 
-def _clean_dag(dag: nx.DiGraph) -> nx.DiGraph:
+def _clean_dag(dag: DAG) -> DAG:
     """Clean the DAG."""
-    for node in dag.nodes:
-        dag.nodes[node].clear()
-    return dag
-
-
-def _style_dag(dag: nx.DiGraph) -> nx.DiGraph:
-    """Style the DAG."""
-    shapes = {name: "hexagon" if "::task_" in name else "box" for name in dag.nodes}
-    nx.set_node_attributes(dag, shapes, "shape")
     return dag
 
 
 def _write_graph(dag: nx.DiGraph, path: Path, layout: str) -> None:
     """Write the graph to disk."""
+    nx = cast("Any", import_optional_dependency("networkx"))
     path.parent.mkdir(exist_ok=True, parents=True)
     graph = nx.nx_agraph.to_agraph(dag)
     graph.draw(path, prog=layout)