mypy crashes when using SQLite cache without an existing cache

[jenstroeger]

Related issues:

• PR that triggered the original issue: chore(deps-dev): update mypy requirement from !=1.20.2,<1.21,>=1.0.0 to >=2.0.0,<2.4 jenstroeger/python-package-template#1238
• Using mypy via pre-commit #13916 (comment)
• Sqlite writes limit parallel scaling #21235
• Default mypy configuration crashes on python build without sqlite #21440

Crash Report

Since SQLite has become the default cache mypy crashes on different platforms and Python versions in various Action runs: v2.1.0 run, v2.2.0 run, and v2.3.0 run. This appears to happen only if the .mypy_cache/ folder is missing; if it exists then mypy runs ok. (I didn’t poke around that folder, though.) See below.

Traceback

error: INTERNAL ERROR -- Please try using mypy master on GitHub:
https://mypy.readthedocs.io/en/stable/common_issues.html#using-a-development-mypy-build
Please report a bug at https://github.com/python/mypy/issues
version: 2.3.0
Traceback (most recent call last):
  File "/path/to/python-package-template/.venv/lib/python3.14/site-packages/mypy/__main__.py", line 16, in console_entry
    main()
    ~~~~^^
  File "mypy/main.py", line 154, in main
  File "mypy/main.py", line 244, in run_build
    res = build.build(sources, options, None, flush_errors, fscache, stdout, stderr)
  File "mypy/build.py", line 385, in build
  File "mypy/build.py", line 1908, in create_metastore
    mds: MetadataStore = SqliteMetadataStore(
  File "mypy/metastore.py", line 200, in __init__
  File "mypy/metastore.py", line 174, in connect_db
    db.execute("PRAGMA journal_mode=WAL")
sqlite3.OperationalError: database is locked

More variations of this error are linked through the above runs.

To Reproduce

I didn’t do nothin’ 🤓

As per PR jenstroeger/python-package-template#1238 this started happening when moving to mypy v2.0.0 which uses the SQLite cache. At first I thought it’s got to do with the new parallel type-checking but whether that’s en/disabled doesn’t actually matter. Whether a .mypy_cache/ already exists does: remove the folder to make mypy crash, or leave the folder in place (even after a crash) and mypy works ok.

Your Environment

I can reproduce this issue reliably locally on macOS Ventura 13.7.8 with Python 3.14.6, and as per linked CI runs above this happens across platforms and Python versions.

• Mypy version used: v2.1.0 and v2.2.0 and v2.3.0
• Mypy command-line flags: --explicit-package-bases --show-traceback --config-file pyproject.toml
• Mypy configuration options from pyproject.toml (via pre-commit):

  [tool.mypy]
  # mypy_path =
  # exclude =
  # num_workers = 
  show_error_codes = true
  show_column_numbers = true
  pretty = true
  show_traceback = true
  check_untyped_defs = true
  incremental = false
  strict = true
  warn_return_any = true
  warn_redundant_casts = true
  warn_unreachable = true
  warn_unused_configs = true
  warn_unused_ignores = true
  disallow_any_explicit = true
  disallow_untyped_calls = true
  disallow_untyped_defs = true
  disallow_incomplete_defs = true
  disallow_untyped_decorators = true
  # disable_error_code =
  # allow_redefinition =
  no_sqlite_cache = true
  
  [[tool.mypy.overrides]]
  module = [
    "pytest.*",
  ]
  ignore_missing_imports = true

• Python version used: from 3.10 through 3.14
• Operating system and version: Linux, Windows, macOS

[ilevkivskyi]

To clarify: does this happen only when you use pre-commit, or it also happens when you run mypy directly (i.e. without pre-commit)?

[jenstroeger]

@ilevkivskyi that is correct.

And after reading through issue #13916 and adjusting my use of mypy and pre-commit accordingly in PR jenstroeger/python-package-template#1248 the issue seems to be gone. I’ll do some more testing over the weekend…

[ilevkivskyi]

OK, I would then propose to close this as a duplicate of #13916 We can continue discussion there (if needed).