☆ Exercise 2

Resolve the conflicts generated in the previous exercise when we created three pull requests: - feature/na-handling (modifies existing functions + stats_demo.py) - feature/add-stdev (adds new function + stats_demo.py) - style/black-formatting (reformats all files)

Step 1. Attempt to Merge the Second PR

Now try to merge the second PR (e.g., feature/add-stdev):

❌ GitHub shows: “This branch has conflicts that must be resolved”

Why conflicts exist:

• stats_lib.py: If Group A modified existing functions and Group B added a new function, there might be conflicts in imports or at the end of the file
• stats_demo.py: Both groups added different test code to the same file

Step 2. Resolve Conflicts for Second PR

The person who created feature/add-stdev should:

Step 1: Update your local repository

git checkout main
git pull origin main  # Get the merged changes from feature/na-handling

Step 2: Merge main into your feature branch

git checkout feature/add-stdev
git merge main

Output might show:

Auto-merging stats_lib.py
Auto-merging stats_demo.py
CONFLICT (content): Merge conflict in stats_demo.py
Automatic merge failed; fix conflicts and then commit the result.

Step 3: Resolve stats_demo.py conflicts

Open stats_demo.py and look for conflict markers:

<<<<<<< HEAD
# Your std_dev demo code
print("\n--- Standard Deviation ---")
print(f"Standard deviation: {std_dev(data)}")
=======
# The NA handling demo code from main
test_data_with_na = [1, 2, None, 4, 5, None, 7]
print(f"Mean (with NAs): {mean(test_data_with_na)}")
print(f"Variance (with NAs): {variance(test_data_with_na)}")
>>>>>>> main

Resolution: Keep BOTH sets of changes:

# NA handling demo
test_data_with_na = [1, 2, None, 4, 5, None, 7]
print(f"Mean (with NAs): {mean(test_data_with_na)}")
print(f"Variance (with NAs): {variance(test_data_with_na)}")

# Standard deviation demo
print("\n--- Standard Deviation ---")
print(f"Standard deviation: {std_dev(data)}")
print(f"This measures spread from the mean")

Step 4: Check if stats_lib.py has conflicts

If the std_dev function uses variance (which now has NA handling), you might need to update it or ensure it works correctly with the new NA-handling code.

Step 5: Test everything works

python stats_demo.py

Step 6: Commit the resolution

git add stats_demo.py stats_lib.py
git commit -m "Resolve merge conflict: combine NA handling with std_dev function"
git push origin feature/add-stdev

Step 7: Merge the PR on GitHub

The PR should now show no conflicts. Repository Owner can merge it!

✅ Second PR merged successfully!

Step 3: Resolve Conflicts for Third PR (Black Formatting)

Now the final PR (style/black-formatting) will definitely have conflicts because it reformatted code that has since been modified.

The person who created style/black-formatting should:

Step 1: Update and merge

git checkout main
git pull origin main
git checkout style/black-formatting
git merge main

Output:

Auto-merging stats_lib.py
CONFLICT (content): Merge conflict in stats_lib.py
Auto-merging stats_demo.py
CONFLICT (content): Merge conflict in stats_demo.py
Automatic merge failed; fix conflicts and then commit the result.

Step 2: Resolve conflicts in stats_lib.py

You’ll see conflicts where Black’s formatting meets the new code:

<<<<<<< HEAD
# Black-formatted original code
def mean(data):
    return sum(data) / len(data)
=======
# New NA-handling version with different formatting
def mean(data):
    """Calculate mean, ignoring None values."""
    clean_data = [x for x in data if x is not None]
    
    if not clean_data:
        raise ValueError("No valid data points")
    
    return sum(clean_data) / len(clean_data)

def std_dev(data):
    """Calculate standard deviation..."""
    return math.sqrt(variance(data))
>>>>>>> main

Resolution strategy: Keep ALL the new functionality from main (NA handling + std_dev), then re-apply Black:

1. Accept all changes from main (the NA handling and std_dev function)
2. Remove conflict markers
3. Run Black again to format everything

Step 3: Accept main’s version and re-format

# For each conflicted file, accept their changes
git checkout --theirs stats_lib.py stats_demo.py

# Re-apply Black to format the new code
black .

Step 4: Verify everything still works

python stats_demo.py

You should see output for: - Original statistics - NA handling tests - Standard deviation calculation

Step 5: Commit the resolution

git add .
git commit -m "Resolve merge conflicts: apply Black formatting to updated codebase"
git push origin style/black-formatting

Step 6: Merge the final PR

Repository Owner can now merge the last PR!

✅ All three PRs successfully merged!