Simplify graph in get_topological_weights.

news/10557.bugfix.rst

@@ -0,0 +1 @@
+Optimize installation order calculation to improve performance when installing requirements that form a complex dependency graph with a large amount of edges.

src/pip/_internal/resolution/resolvelib/resolver.py

@@ -177,6 +177,10 @@ def get_installation_order(
         """
         assert self._result is not None, "must call resolve() first"
 
+        if not req_set.requirements:
+            # Nothing is left to install, so we do not need an order.
+            return []
+
         graph = self._result.graph
         weights = get_topological_weights(
             graph,
@@ -227,6 +231,34 @@ def visit(node: Optional[str]) -> None:
         last_known_parent_count = weights.get(node, 0)
         weights[node] = max(last_known_parent_count, len(path))
 
+    # Simplify the graph, pruning leaves that have no dependencies.
+    # This is needed for large graphs (say over 200 packages) because the
+    # `visit` function is exponentially slower then, taking minutes.
+    # See https://github.com/pypa/pip/issues/10557
+    # We will loop until we explicitly break the loop.
+    while True:
+        leaves = set()
+        for key in graph:
+            if key is None:
+                continue
+            for _child in graph.iter_children(key):
+                # This means we have at least one child
+                break
+            else:
+                # No child.
+                leaves.add(key)
+        if not leaves:
+            # We are done simplifying.
+            break
+        # Calculate the weight for the leaves.
+        weight = len(graph) - 1
+        for leaf in leaves:
+            weights[leaf] = weight
+        # Remove the leaves from the graph, making it simpler.
+        for leaf in leaves:
+            graph.remove(leaf)
+
+    # Visit the remaining graph.
     # `None` is guaranteed to be the root node by resolvelib.
     visit(None)
 

tests/unit/resolution_resolvelib/test_resolver.py

@@ -115,7 +115,7 @@ def test_new_resolver_get_installation_order(
                 ("three", "four"),
                 ("four", "five"),
             ],
-            {None: 0, "one": 1, "two": 1, "three": 2, "four": 3, "five": 4},
+            {None: 0, "five": 5, "four": 4, "one": 4, "three": 2, "two": 1},
         ),
         (
             "linear",