Ty Type Checking

ty runs in CI as an informational check to evaluate its usefulness vs mypy. If you see ty annotations in your PR:

• 👀 Review the feedback - ty often catches real type issues
• 📊 Share your experience in #team-devex
• ⚠️ ty warnings are informational and don't block CI

The baseline system filters out 567 pre-existing errors so you only see new issues introduced by your changes.

Manual Usage

./bin/ty.py check path/to/file.py    # Check specific files
./bin/ty.py check posthog ee         # Check directories

Ty vs mypy: Fast trial vs authoritative checking

ty is currently in trial mode:

• ⚡ Extremely fast (~10-100x faster than mypy)
• 🧪 Alpha software (v0.0.1a22 - expect edge cases)
• 📢 Runs in CI only (non-blocking) to gather feedback
• 🚨 Uses GitHub problem matcher to show warnings inline

mypy remains the authoritative type checker:

• 🎯 More mature and comprehensive
• 🐌 Slower but thorough
• ✅ Final source of truth (runs in CI and blocks on errors)
• 📍 Runs in CI and recommended for local deep checks

This trial helps us evaluate whether ty should become a blocking check in the future.

Baseline Management

Two Separate Baselines

• mypy-baseline.txt - Maintained by mypy (~1287 errors)
• ty-baseline.txt - Maintained by ty (567 diagnostics)

ty maintains its own baseline because it reports different errors than mypy. The baseline contains pre-existing errors that won't trigger warnings in CI.

Baseline Contents

The ty-baseline.txt contains:

• 91 redundant-cast (easy cleanup opportunities)
• 96 possibly-unbound-attribute (null safety checks)
• 119 missing-argument (real bugs requiring investigation)
• 52 deprecated warnings
• Plus other categories

About 50% are real bugs, 25% safety improvements, 25% trivial cleanups.

Updating ty-baseline

When you fix ty errors across the codebase, update the baseline:

./bin/ty.py sync
git add ty-baseline.txt
git commit -m "chore: update ty baseline after fixing type errors"

The sync command runs ty on all Python directories and updates ty-baseline.txt with the current state.

How Filtering Works in CI

1. CI runs ty on your changed files
2. ty finds errors (both old and new)
3. Baseline filter removes errors already in ty-baseline.txt
4. Only new errors you introduced are shown as warnings
5. CI always passes (ty is informational only)