From 5b0cd60dc0c8d83067c4ed95fe50dfe0145c379b Mon Sep 17 00:00:00 2001
From: dennis <dnguyen@nextdoor.com>
Date: Thu, 6 Aug 2026 15:41:24 -0700
Subject: [PATCH 1/2] Fix dmypy traversal of missing graph dependencies

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
---
 mypy/dmypy_server.py       |  8 ++++++--
 test-data/unit/daemon.test | 34 ++++++++++++++++++++++++++++++++++
 2 files changed, 40 insertions(+), 2 deletions(-)

diff --git a/mypy/dmypy_server.py b/mypy/dmypy_server.py
index 066fbf9bed2d6..fe16536fa403d 100644
--- a/mypy/dmypy_server.py
+++ b/mypy/dmypy_server.py
@@ -769,7 +769,7 @@ def find_reachable_changed_modules(
                 state = graph[nxt.module]
                 ancestors = state.ancestors or []
                 for dep in state.dependencies + ancestors:
-                    if dep not in seen:
+                    if dep not in seen and dep in graph:
                         seen.add(dep)
                         worklist.append(BuildSource(graph[dep].path, graph[dep].id, followed=True))
         return changed, new_files
@@ -779,7 +779,11 @@ def direct_imports(
     ) -> list[BuildSource]:
         """Return the direct imports of module not included in seen."""
         state = graph[module[0]]
-        return [BuildSource(graph[dep].path, dep, followed=True) for dep in state.dependencies]
+        return [
+            BuildSource(graph[dep].path, dep, followed=True)
+            for dep in state.dependencies
+            if dep in graph
+        ]
 
     def find_added_suppressed(
         self, graph: mypy.build.Graph, seen: set[str], search_paths: SearchPaths
diff --git a/test-data/unit/daemon.test b/test-data/unit/daemon.test
index 448af5d080029..29046fcbd5a8c 100644
--- a/test-data/unit/daemon.test
+++ b/test-data/unit/daemon.test
@@ -802,3 +802,37 @@ Found 1 error in 1 file (checked 1 source file)
 [file test.py]
 from xml.etree.ElementTree import Element
 1 + 'a'
+
+[case testDaemonRecheckMissingGraphDependency]
+$ dmypy start
+Daemon started
+$ dmypy check src/
+src/pkg/example.py:2: error: Name "x" already defined on line 1  [no-redef]
+Found 1 error in 1 file (checked 2 source files)
+== Return code: 1
+$ dmypy recheck
+src/pkg/example.py:2: error: Incompatible import of "x" (imported name has type "int", local name has type "str")  [assignment]
+Found 1 error in 1 file (checked 53 source files)
+== Return code: 1
+$ dmypy recheck
+src/pkg/example.py:2: error: Incompatible import of "x" (imported name has type "int", local name has type "str")  [assignment]
+Found 1 error in 1 file (checked 53 source files)
+== Return code: 1
+[file mypy.ini]
+\[mypy]
+follow_imports = normal
+mypy_path = src
+files = src/pkg/
+exclude = (?x)(
+    \./.*
+    | tests
+  )
+\[mypy-pkg.tests.*]
+follow_imports = skip
+[file src/pkg/__init__.py]
+[file src/pkg/example.py]
+x = "1"
+from pkg.tests.test_something import x
+[file src/pkg/tests/__init__.py]
+[file src/pkg/tests/test_something.py]
+x = 1

From 7e0ac76c25aed1779c26908a67abddfa61832ed0 Mon Sep 17 00:00:00 2001
From: dennis <dnguyen@nextdoor.com>
Date: Thu, 6 Aug 2026 15:53:42 -0700
Subject: [PATCH 2/2] Avoid typeshed-dependent file count in daemon regression
 test

The "checked N source files" summary line varies with the bundled typeshed
version, which would make the test fail spuriously after a typeshed sync.
Start the daemon with --no-error-summary so the test asserts only on the
diagnostics that the fix actually affects.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
---
 test-data/unit/daemon.test | 7 +++----
 1 file changed, 3 insertions(+), 4 deletions(-)

diff --git a/test-data/unit/daemon.test b/test-data/unit/daemon.test
index 29046fcbd5a8c..5fb4eaca55d88 100644
--- a/test-data/unit/daemon.test
+++ b/test-data/unit/daemon.test
@@ -804,19 +804,18 @@ from xml.etree.ElementTree import Element
 1 + 'a'
 
 [case testDaemonRecheckMissingGraphDependency]
-$ dmypy start
+-- Dropping pkg.tests from the graph leaves a stale ancestors edge behind, which
+-- a later recheck used to dereference unconditionally (KeyError: 'pkg.tests').
+$ dmypy start -- --no-error-summary
 Daemon started
 $ dmypy check src/
 src/pkg/example.py:2: error: Name "x" already defined on line 1  [no-redef]
-Found 1 error in 1 file (checked 2 source files)
 == Return code: 1
 $ dmypy recheck
 src/pkg/example.py:2: error: Incompatible import of "x" (imported name has type "int", local name has type "str")  [assignment]
-Found 1 error in 1 file (checked 53 source files)
 == Return code: 1
 $ dmypy recheck
 src/pkg/example.py:2: error: Incompatible import of "x" (imported name has type "int", local name has type "str")  [assignment]
-Found 1 error in 1 file (checked 53 source files)
 == Return code: 1
 [file mypy.ini]
 \[mypy]