Automated static type checking with MyPy

[lukasjuhrich]

Many bugs in the past could have been avoided by some simple type checking. This is a Meta-Issue tracking the progress and to use as a reference in commits which extend our typing efforts.

Sub-Issues / milestones

• 557

• [ ] Make mypy pass on every file (null checks disabled)
  • [x] pycroft.lib
  • [ ] pycroft.model
  • [x] pycroft.helpers
  • [x] pycroft.helpers.date
  • [x] pycroft.helpers.errorcode
  • [x] pycroft.helpers.facilities
  • [x] pycroft.helpers.i18n
  • [x] pycroft.helpers.interval
  • [x] pycroft.helpers.net
  • [x] pycroft.helpers.printing
  • [x] pycroft.helpers.task
  • [x] pycroft.helpers.user
  • [x] pycroft.helpers.utc
  • [x] web
  • [x] ldap_sync
  • [ ] everything else
• [ ] Disallow untyped function signatures
  • [x] pycroft.lib
  • [x] pycroft.lib.address
  • [x] pycroft.lib.exc
  • [x] pycroft.lib.facilities
  • [x] pycroft.lib.finance
  • [x] pycroft.lib.host
  • [x] pycroft.lib.infrastructure
  • [x] pycroft.lib.logging
  • [x] pycroft.lib.mail
  • [x] pycroft.lib.membership
  • [x] pycroft.lib.net
  • [x] pycroft.lib.search
  • [x] pycroft.lib.stats
  • [x] pycroft.lib.swdd
  • [x] pycroft.lib.task
  • [x] pycroft.lib.traffic
  • [x] pycroft.lib.user
  • [x] pycroft.lib.user_deletion
  • [ ] pycroft.model
  • [ ] pycroft.helpers
  • [x] pycroft.helpers.date
  • [x] pycroft.helpers.errorcode
  • [x] pycroft.helpers.facilities
  • [ ] pycroft.helpers.i18n
  • [ ] pycroft.helpers.interval
  • [x] pycroft.helpers.net
  • [x] pycroft.helpers.printing
  • [x] pycroft.helpers.task
  • [x] pycroft.helpers.user
  • [x] pycroft.helpers.utc
  • [ ] web
  • [x] ldap_sync
  • [ ] everything else
• [ ] Make mypy pass with null checks (i.e. no_strict_optional=false) (if viable; might be quite a hassle)
  • [ ] pycroft.lib
  • [ ] pycroft.model
  • [ ] web
  • [ ] everything else
• [ ] Use a MoneyAmount NewType for the sqlalchemy Money type decorator to not just have Decimal

Acceptance criteria

There is a mypy CI checking at least pycroft and web (with --check-untyped-defs).

[lukasjuhrich]

Just because I don't know where else to write this, the following is worth noting about how to type the SQLAlchemy models:

Above, the ultimate class-level datatypes of id, name and other_name will be introspected as Mapped[Optional[int]], Mapped[Optional[str]] and Mapped[Optional[str]]. The types are by default always considered to be Optional, even for the primary key and non-nullable column. The reason is because while the database columns “id” and “name” can’t be NULL, the Python attributes id and name most certainly can be None without an explicit constructor:

(from the docs)

[lukasjuhrich]

So, fundamentally, SQLAlchemy/mypy is not able to distinguish between

1. a newly constructed User
2. a User fetched from a database.

This discrepancy is what statically typed languages usually avoid with the builder pattern.

[lukasjuhrich]

Also relevant: https://mypy.readthedocs.io/en/stable/common_issues.html

[lukasjuhrich]

See also these notes

[lukasjuhrich]

web enabled by #661