# Module 07: Git Stash - Temporary Storage

## Learning Objectives

By the end of this module, you will:
- Understand what git stash is and when to use it
- Temporarily save work without committing
- Switch between branches without losing uncommitted changes
- Manage multiple stashes
- Apply and remove stashed changes
- Understand the difference between stash pop and stash apply

## Challenge Description

You're working on a feature when your teammate reports a critical bug in production. You need to switch to the main branch to fix it immediately, but your current work is incomplete and not ready to commit.

Your task is to:
1. Stash your incomplete work
2. Switch to the main branch
3. Fix the urgent bug and commit it
4. Return to your feature branch
5. Restore your stashed changes
6. Complete your feature and commit it

## Key Concepts

### What is Git Stash?

Git stash temporarily saves your uncommitted changes (both staged and unstaged) and reverts your working directory to match the HEAD commit. Think of it as a clipboard for your changes.

### When to Use Stash

Use stash when you need to:
- Switch branches but have uncommitted changes
- Pull updates from remote but have local modifications
- Quickly test something on a clean working directory
- Save work temporarily without creating a commit
- Context-switch between tasks

### Stash vs Commit

**Stash:**
- Temporary storage
- Not part of project history
- Can be applied to different branches
- Easy to discard if not needed
- Local only (not pushed to remote)

**Commit:**
- Permanent part of history
- Creates a snapshot in the project timeline
- Associated with a specific branch
- Should be meaningful and complete
- Can be pushed to remote

### The Stash Stack

Git stash works like a stack (LIFO - Last In, First Out):
```
stash@{0}  <- Most recent stash (top of stack)
stash@{1}
stash@{2}  <- Oldest stash
```

You can have multiple stashes and apply any of them.

## Setup

Run the setup script to create the challenge environment:

```pwsh
.\setup.ps1
```

This creates a `challenge/` directory where you're working on a login feature with uncommitted changes, and a critical bug needs fixing on main.

## Your Task

### The Scenario

You're working on a login feature on the `feature-login` branch. Your work is incomplete (has TODOs), so it's not ready to commit.

Suddenly, your teammate reports a **critical security bug** in production! You need to:
1. Temporarily save your incomplete work
2. Switch to the main branch
3. Fix the urgent bug
4. Return to your feature and continue working

### Step-by-Step Instructions

1. **Navigate to the challenge directory:**
   ```pwsh
   cd challenge
   ```

2. **Check your current status:**
   ```pwsh
   git status
   ```
   You should see modified `login.py` (uncommitted changes)

3. **Stash your work with a message:**
   ```pwsh
   git stash push -m "WIP: login feature"
   ```
   
4. **Verify working directory is clean:**
   ```pwsh
   git status
   ```
   Should say "nothing to commit, working tree clean"

5. **Switch to main branch:**
   ```pwsh
   git switch main
   ```

6. **Open app.py and find the bug:**
   ```pwsh
   cat app.py
   ```
   Look for the comment "# BUG: This allows unauthenticated access!"

7. **Fix the bug** by editing app.py:
   - Remove the buggy comment line
   - You can leave the implementation as-is or improve it
   - The important thing is removing the comment that says "allows unauthenticated access"

8. **Commit the fix:**
   ```pwsh
   git add app.py
   git commit -m "Fix critical security bug"
   ```

9. **Switch back to your feature branch:**
   ```pwsh
   git switch feature-login
   ```

10. **Restore your stashed work:**
    ```pwsh
    git stash pop
    ```
    This applies the stash and removes it from the stash stack

11. **Complete the TODOs in login.py:**
    - Open login.py in your editor
    - Complete the login method (verify password and return session)
    - Add a logout method
    - Remove all TODO comments

12. **Commit your completed feature:**
    ```pwsh
    git add login.py
    git commit -m "Complete login feature"
    ```

13. **Verify your solution:**
    ```pwsh
    ..\verify.ps1
    ```

## Key Stash Commands

### Basic Operations

```pwsh
# Stash current changes with a message
git stash push -m "description"

# Stash without a message (not recommended)
git stash

# Stash including untracked files
git stash -u

# List all stashes
git stash list

# Show what's in the most recent stash
git stash show

# Show full diff of stash
git stash show -p
```

### Applying Stashes

```pwsh
# Apply most recent stash and remove it (RECOMMENDED)
git stash pop

# Apply most recent stash but keep it in stack
git stash apply

# Apply a specific stash
git stash apply stash@{1}

# Apply a specific stash by number
git stash apply 1
```

### Managing Stashes

```pwsh
# Drop (delete) the most recent stash
git stash drop

# Drop a specific stash
git stash drop stash@{1}

# Clear all stashes
git stash clear

# Create a new branch from a stash
git stash branch new-branch-name
```

## Understanding Stash vs Pop vs Apply

### Stash Pop (Recommended)
```pwsh
git stash pop
```
- Applies the stash to your working directory
- **Removes** the stash from the stack
- Use this most of the time

### Stash Apply (Keep Stash)
```pwsh
git stash apply
```
- Applies the stash to your working directory
- **Keeps** the stash in the stack
- Useful if you want to apply the same changes to multiple branches

### When to Use Which

**Use `pop` when:**
- You're done with the stash and won't need it again (99% of the time)
- You want to keep your stash list clean

**Use `apply` when:**
- You want to test the same changes on different branches
- You're not sure if you want to keep the stash yet

## Verification

Run the verification script to check your solution:

```pwsh
..\verify.ps1
```

Or from the module directory:

```pwsh
.\verify.ps1
```

The verification will check that:
- ✅ The bug fix commit exists on main
- ✅ Your feature is completed on the feature-login branch
- ✅ All TODOs are removed from login.py
- ✅ No uncommitted changes remain

## Troubleshooting

### "Cannot switch branches - you have uncommitted changes"

**Problem:** Git won't let you switch branches with uncommitted changes.

**Solution:**
```pwsh
# Stash your changes first
git stash push -m "work in progress"

# Now you can switch
git switch other-branch

# When you come back, restore your work
git switch original-branch
git stash pop
```

### "I don't remember what's in my stash"

**Problem:** You stashed something but forgot what it was.

**Solution:**
```pwsh
# List all stashes
git stash list

# Show summary of what changed
git stash show stash@{0}

# Show full diff
git stash show -p stash@{0}
```

### "Stash conflicts when I apply"

**Problem:** Applying a stash causes merge conflicts.

**Solution:**
1. Git marks conflicts in your files with `<<<<<<<` markers
2. Open the files and resolve conflicts manually
3. Stage the resolved files: `git add <file>`
4. If you used `pop`, the stash is automatically dropped
5. If you used `apply`, manually drop it: `git stash drop`

### "I accidentally cleared my stash"

**Problem:** You deleted a stash you still needed.

**Unfortunately:** Stashes are hard to recover once deleted. Lessons learned:
- Use `git stash pop` instead of `git stash drop` when you're unsure
- Use `git stash show -p` to preview before dropping
- Consider committing work instead of stashing for important changes

## Tips for Success

💡 **Always add a message** - `git stash push -m "your message"` helps you remember what you stashed  
💡 **Use pop, not apply** - Pop removes the stash automatically, keeping your stash list clean  
💡 **Stash before pulling** - Avoid merge conflicts when pulling updates  
💡 **Preview before applying** - Use `git stash show -p` to see what's in a stash  
💡 **Stashes are local** - They don't get pushed to remote repositories  
💡 **Clean working directory** - Always verify with `git status` after stashing  

## Common Use Cases

### Quick Branch Switch
```pwsh
# You're working on feature-A
git stash push -m "feature A progress"
git switch hotfix-branch
# Fix the issue, commit
git switch feature-A
git stash pop
```

### Pull with Local Changes
```pwsh
# You have uncommitted changes
git stash push -m "local changes"
git pull
git stash pop
# Resolve conflicts if any
```

### Test Clean State
```pwsh
# Stash changes to test on clean code
git stash push -m "testing clean state"
# Run tests
git stash pop  # Restore your changes
```

## What You've Learned

After completing this module, you understand:

- ✅ Stash temporarily saves uncommitted changes
- ✅ Stash lets you switch contexts without committing
- ✅ `git stash pop` applies and removes the stash
- ✅ `git stash apply` applies but keeps the stash
- ✅ Stashes are local and not pushed to remote
- ✅ Stash is essential for handling interruptions and urgent fixes

**Key Takeaway:** Stash is your "temporary clipboard" for incomplete work. It helps you stay productive when you need to context-switch without making messy "WIP" commits.

## Next Steps

Ready to continue? You've now mastered the essential Git commands for daily development:
- Committing and history
- Branching and merging
- Cherry-picking specific changes
- Safely reverting commits
- Temporarily stashing work

Move on to **Module 08: Multiplayer Git** to practice collaborating with others!

To start over:
```pwsh
.\reset.ps1
```