Branching-Out Demo(Python)

A simple Python project for learning and practice purposes.
This application demonstrates the usage of branches.

Features

• Filters user data from a JSON structure
• Main branch implemented a simple name filter
• Feature branch feature-filter-by-age added an age filter
• Feature branch feature-filter-by-email added an email filter
• The email filter was merged with main by a pull request

---

Run the application

Start the project with:

python|python3 filter_users.py

Git History with Fast-Forward and Merge Commit

This document explains the following Git history step by step:

git log --oneline --graph

* d7ed803 (HEAD -> main, origin/main) code comments added
*   f05a05b Merge pull request #1 from user0815/feature-filter-by-email
|\  
| * 2b3cf4e (origin/feature-filter-by-email, feature-filter-by-email) filter by email added
|/  
* 435faed (origin/feature-filter-by-age, feature-filter-by-age) filter by age added
* 35d4671 basic user filter by name added
* d987b26 first commit

---

1. Initial Commit

* d987b26 first commit

The repository is created.
This is the starting point of the entire history.

main
 |
 v
d987b26
first commit

---

2. Development on main

* 35d4671 basic user filter by name added
* d987b26 first commit

A second commit is added directly to the main branch.

d987b26
first commit
   |
   v
35d4671
basic user filter by name added

At this point, the history is completely linear.

---

3. Feature Branch feature-filter-by-age

A new feature branch is created from main:

git checkout -b feature-filter-by-age

A new commit is added on this branch:

435faed filter by age added

The history now looks like this:

d987b26
first commit
   |
   v
35d4671
basic user filter by name added
   |
   v
435faed
filter by age added

Since no new commits were added to main in the meantime, this branch can later be integrated using a fast-forward merge.

---

4. Fast-Forward Merge of feature-filter-by-age

The branch is merged into main:

git checkout main
git merge feature-filter-by-age

Git recognizes:

main is a direct ancestor of feature-filter-by-age

Therefore, no additional merge commit is required.

Instead, Git simply moves the main branch pointer forward.

Before fast-forward:

d987b26 --- 35d4671        main
              \
               435faed    feature-filter-by-age

After fast-forward:

d987b26 --- 35d4671 --- 435faed
                         main
                         feature-filter-by-age

Important:

• No new commit is created.
• The history remains linear.
• main now points to the same commit as feature-filter-by-age.

---

5. New Feature Branch feature-filter-by-email

Another feature branch is created from the current state:

git checkout -b feature-filter-by-email

A new commit is added on this branch:

2b3cf4e filter by email added

The history now looks like this:

d987b26 --- 35d4671 --- 435faed
                              \
                               2b3cf4e
                               feature-filter-by-email

The commit 2b3cf4e initially exists only on the feature branch.

---

6. Merge via Pull Request

The branch feature-filter-by-email is later merged into main using a pull request.

This creates a dedicated merge commit:

f05a05b Merge pull request #1 from user0815/feature-filter-by-email

The history now looks like this:

d987b26 --- 35d4671 --- 435faed -------- f05a05b
                              \         /
                               2b3cf4e

The merge commit f05a05b combines two development lines:

1. the existing main history
2. the feature branch feature-filter-by-email

Therefore, this commit has two parent commits.

---

7. Merge Visualization in Git Log

The merge is displayed in git log --oneline --graph like this:

*   f05a05b Merge pull request #1 from user0815/feature-filter-by-email
|\  
| * 2b3cf4e filter by email added
|/  
* 435faed filter by age added

Meaning of the symbols:

*    Commit
|    Connection line
\    Branch split
/    Merge back into main line
|\   Merge commit with two parents

---

8. Additional Commit on main

After the merge, development continues directly on main.

d7ed803 code comments added

The final history becomes:

d987b26 --- 35d4671 --- 435faed -------- f05a05b --- d7ed803
                              \         /
                               2b3cf4e

HEAD, main, and origin/main now point to the latest commit:

d7ed803 (HEAD -> main, origin/main)

This means:

• The current local branch is main
• HEAD points to main
• The local main branch is synchronized with origin/main

---

9. Complete Git Graph

* d7ed803 (HEAD -> main, origin/main) code comments added
*   f05a05b Merge pull request #1 from user0815/feature-filter-by-email
|\  
| * 2b3cf4e (origin/feature-filter-by-email, feature-filter-by-email) filter by email added
|/  
* 435faed (origin/feature-filter-by-age, feature-filter-by-age) filter by age added
* 35d4671 basic user filter by name added
* d987b26 first commit

---

10. Summary

This history contains two different integration strategies.

Fast-Forward Merge

Used for feature-filter-by-age:

main was simply moved forward.

Characteristics:

• no merge commit
• linear history
• clean and simple
• possible only if main has not changed since the branch was created

---

Real Merge Commit

Used for feature-filter-by-email:

A dedicated merge commit was created.

Characteristics:

• visible branch structure
• dedicated merge commit
• two parent commits
• common for GitHub pull requests

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11. Key Takeaway

A fast-forward merge is simply moving the branch pointer forward.

A merge commit is a real combination of two separate development histories.