Root Cause: You're Running Tools Outside of pre-commit
The issue is in your reproduction steps - you're installing and running mypy separately:

python -m pip install mypy # ❌ This is the problem
mypy . # ❌ This bypasses pre-commit config
When you run mypy . directly, it uses:

Whatever mypy version you installed (likely latest)
Default mypy configuration (not the repo's)
All files in the directory
The CI uses pre-commit hooks which:

Pin specific tool versions in .pre-commit-config.yaml
Use the project's mypy configuration
Only check relevant files with specific flags
The Fix: Only Use pre-commit
git clone https://github.com/TheAlgorithms/Python
cd Python

python -m venv .venv
source .venv/bin/activate

pip install --upgrade pip
pip install pre-commit # Only install pre-commit
pre-commit install

This is all you need - matches CI exactly

pre-commit run --all-files
Don't install mypy, ruff, or any other tools separately. pre-commit handles everything.

Verifying Your Setup Matches CI
Check tool versions:

cat .pre-commit-config.yaml
Look for the rev: values - these are the exact versions CI uses.

Check Python version:

cat .github/workflows/*.yml | grep python-version
Make sure your venv uses the same Python version.

Verify pre-commit is working:

pre-commit run --all-files --verbose
This should show which hooks are running and their versions.

If You See Errors in Files You Didn't Modify
First, verify main branch is actually clean:

git checkout main
git pull origin main
pre-commit run --all-files
If main is clean but you still see errors:

Your branch might be outdated:

git fetch origin
git rebase origin/main
Pre-commit cache might be stale:

pre-commit clean
pre-commit install --install-hooks
pre-commit run --all-files
For your PR, only check your changes:

Stage your changes

git add path/to/your/file.py

Run checks only on staged files

pre-commit run
Answering Your Questions
Q: What is the correct way to align local tool versions with CI?

A: Only use pre-commit. Don't install mypy, ruff, or other tools separately. The .pre-commit-config.yaml file pins all versions to match CI exactly.

Q: If type errors appear outside the files I changed, what should I do?

A:

Don't fix them in your PR - keep your PR focused on your changes
Verify main is clean - if main has errors, that's a separate issue
For your PR - use pre-commit run (without --all-files) to only check staged files
If you find repo-wide issues - open a separate issue/PR to discuss with maintainers
Pro Tip: Pre-commit Workflow

Before making changes

git checkout -b my-feature
pre-commit run --all-files # Verify main is clean

Make your changes

... edit files ...

Check only your changes

git add .
pre-commit run # Only checks staged files

If all passes, commit

git commit -m "Your changes"
This ensures your local checks match CI behavior exactly, and you only deal with errors in files you actually modified.