Test-Guided Migration Assistant

Automatically update a codebase to a new language version, framework version, or library update while ensuring all tests continue to pass.

Workflow

1. Establish Baseline

Before starting migration, establish the current state:

# Run full test suite
<test_command>

# Record baseline
# - Total tests: X
# - Passing: Y
# - Failing: Z (should be 0 or known failures)
# - Coverage: N%

Document current state:

Current versions (language, framework, dependencies)
Test results
Any known issues

2. Understand Migration Context

Gather information:

Target version (e.g., Python 3.12, React 18, Django 4.2)
Migration scope (single library vs. full stack)
Breaking changes expected

Consult migration guides:

Official migration documentation
Changelog/release notes
Community migration guides
See migration_scenarios.md for common patterns

Identify risks:

Deprecated API usage
Known breaking changes
Custom integrations affected
Performance implications

3. Perform Migration

Update dependencies:

Incremental approach (recommended):

# Update one dependency at a time
npm install package@new-version
# or
pip install package==new-version

# Run tests immediately
<test_command>

Batch approach (for experienced migrations):

# Update all dependencies
npm update
# or
pip install -U -r requirements.txt

# Run tests
<test_command>

Update configuration:

Update build configuration
Update CI/CD configuration
Update environment files
Update Docker images

4. Analyze Test Failures

Run tests and capture output:

# Pytest
pytest -v > test_output.txt 2>&1

# Jest
npm test > test_output.txt 2>&1

# Other frameworks
<test_command> > test_output.txt 2>&1

Use analysis script:

python scripts/analyze_test_failures.py test_output.txt

The script categorizes failures into:

🔴 Import/Module Errors (CRITICAL - fix first)
🟠 API Signature Errors (HIGH priority)
🟡 Type Errors (MEDIUM priority)
🟡 Configuration Errors (MEDIUM priority)
🟢 Behavior Changes (review carefully)
🔵 Deprecation Warnings (fix when possible)

5. Fix Issues Systematically

Follow priority order from analysis. See migration_strategy.md for detailed strategies.

Fix import errors first:

# Example: Module moved
# Old
from collections import Mapping

# New
from collections.abc import Mapping

Fix API signature errors:

# Example: Parameter renamed
# Old
result = function(arg1, old_param=value)

# New
result = function(arg1, new_param=value)

Fix type errors:

// Example: Stricter types
// Old
function process(value: string) { }

// New (handle undefined)
function process(value: string | undefined) {
  if (value === undefined) return;
  // ...
}

Handle behavior changes:

# Example: Changed defaults
# Old behavior: returns None on error
# New behavior: raises exception

# Add error handling
try:
    result = function()
except NewException:
    result = None  # Preserve old behavior

After each fix category:

# Run tests
<test_command>

# Commit if tests pass
git add .
git commit -m "Fix: migration import errors"

6. Verify Behavior Preservation

Run full test suite multiple times:

# Catch flaky tests
for i in {1..3}; do
  echo "Run $i"
  <test_command>
done

Verify coverage unchanged:

# Generate coverage report
<coverage_command>

# Compare to baseline
# Should be same or better

Check for warnings:

# Python
python -W all -m pytest

# Node.js
node --trace-warnings test

# Look for deprecation warnings

Success criteria:

7. Document Migration

Create migration summary:

MIGRATION SUMMARY
=================

Migration: Python 3.8 → Python 3.12

Changes Made:
- Updated collections imports (Mapping → collections.abc.Mapping)
- Fixed asyncio.coroutine → async def
- Updated type hints for stricter checking
- Removed distutils usage

Test Results:
- Before: 156/156 passing
- After: 156/156 passing
- Coverage: 87% (unchanged)

Breaking Changes Addressed:
1. Collections ABC moved
2. asyncio.coroutine removed
3. Type checking stricter

Deprecation Warnings:
- None remaining

Key Principles

Tests Define Correctness: If tests pass, behavior is preserved.

Fix by Priority: Import errors block everything - fix first. Then API errors, then types, then behavior.

Incremental Changes: Fix one category, test, commit. Don't accumulate many changes.

Never Modify Tests: Fix the code to make tests pass. Don't change tests to match new behavior (unless behavior change is intentional and documented).

Fail Fast: Run tests immediately after migration attempt. Don't wait.

Common Migration Patterns

Python 2 → 3

print statement → print() function
xrange() → range()
.iteritems() → .items()
except Exception, e: → except Exception as e:

React 16 → 18

ReactDOM.render() → ReactDOM.createRoot().render()
Update for concurrent rendering
Fix useEffect timing issues

Django 2.2 → 4.2

django.conf.urls.url() → django.urls.re_path()
Update ugettext → gettext
Remove USE_L10N setting

Node.js 14 → 20

Update deprecated APIs
Fix OpenSSL changes
Update crypto usage

Helper Script

The analyze_test_failures.py script automates failure categorization:

# Analyze pytest output
python scripts/analyze_test_failures.py test_output.txt

# Analyze Jest output
python scripts/analyze_test_failures.py test_output.txt --format jest

# Auto-detect format
python scripts/analyze_test_failures.py test_output.txt --format auto

Output provides:

Categorized failure list
Priority order for fixes
Example errors from each category
Recommended fix strategies

Rollback Strategy

If migration has too many failures (>20% of tests):

# Revert dependency changes
git checkout HEAD -- package.json package-lock.json
npm install

# Or revert commits
git revert <commit-hash>

# Verify tests pass
<test_command>

Then plan incremental migration or allocate more time.

test-guided-migration-assistant