dmypy 'KeyError' on recheck

Crash Report

Running dmypy multiple times results in a KeyError. The error is always on files we exclude in our mypy.ini.

Traceback

[nick@Mac] ~/workspace/mypy-playground [bug-dmypy-key-error]
$ uv run dmypy start -- --show-traceback
Daemon started
[nick@Mac] ~/workspace/mypy-playground [bug-dmypy-key-error]
$ uv run dmypy check src/
src/mypy_playground/example.py:3: error: Name "x" already defined on line 1  [no-redef]
Found 1 error in 1 file (checked 2 source files)
[nick@Mac] ~/workspace/mypy-playground [bug-dmypy-key-error]
$ uv run dmypy recheck
src/mypy_playground/example.py:3: error: Incompatible import of "x" (imported name has type "int", local name has type "str")  [assignment]
Found 1 error in 1 file (checked 61 source files)
[nick@Mac] ~/workspace/mypy-playground [bug-dmypy-key-error]
$ uv run dmypy recheck
Daemon crashed!
Traceback (most recent call last):
  File "mypy/dmypy_server.py", line 237, in serve
  File "mypy/dmypy_server.py", line 286, in run_command
  File "mypy/dmypy_server.py", line 408, in cmd_recheck
  File "mypy/dmypy_server.py", line 629, in fine_grained_increment_follow_imports
  File "mypy/dmypy_server.py", line 768, in find_reachable_changed_modules
KeyError: 'mypy_playground.tests'

To Reproduce
Pushed example source code to https://github.com/njgrisafi/mypy-playground/ on branch bug-dmypy-key-error

See the README https://github.com/njgrisafi/mypy-playground/tree/bug-dmypy-key-error

Your Environment

• Mypy version used: 1.18.2 & 1.16.0
• Mypy command-line flags: dmypy start, dmypy check src/ and dmypy recheck
• Mypy configuration options from mypy.ini (and other config files):

[mypy]
check_untyped_defs = True
ignore_missing_imports = True
no_implicit_optional = True
allow_untyped_globals = True
local_partial_types = True
enable_error_code = ignore-without-code

follow_imports = normal
warn_unused_configs = True
warn_redundant_casts = True
no_warn_no_return = True
disallow_untyped_defs = False
disable_error_code =
  has-type,
  annotation-unchecked,
  misc

mypy_path = src
files = src/mypy_playground/

exclude = (?x)(
    \./.*  # dot-prefixed files
    | tests
  ) # (intentional whitespace, otherwise it errors)


[mypy-tests.*,*.tests.*,*.unit_tests.*,*.unit_test.*,*.integration_tests.*,conftest_integration]
ignore_errors = True
follow_imports = skip

• Python version used: 3.12.11
• Operating system and version: macOS Tahoe 26.1

I suspect it can be related to https://github.com/python/mypy/pull/18906

relates to https://github.com/python/mypy/issues/18755

Beginne mit der Reproduktion im Branch bug-dmypy-key-error und lies dann in mypy/dmypy_server.py den Bereich um cmd_recheck, fine_grained_increment_follow_imports und find_reachable_changed_modules. Führe die wiederholte dmypy-recheck-Sequenz mit dem gezeigten mypy.ini-Ausschluss erneut aus. Erledigt ist die Aufgabe, wenn der Daemon bei den ausgeschlossenen Tests nicht mehr abstürzt und die Regression durch einen geeigneten Test abgedeckt ist.