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In this lesson, you’ll learn how to leverage AI (via Composer) to debug a Flask application. We’ll capture runtime errors in the browser or terminal, feed them into Composer alongside the relevant code, and apply the suggested fixes—all while applying your own programmer intuition.

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1. Reproducing & Resolving a Jinja2 Template Error

  1. Start your Flask development server and navigate to the login page.
  2. Log in with valid credentials.
The image shows a login page for a "Task Manager" application with fields for username and password. The browser window is open alongside a code editor displaying project files.
  1. After logging in, you may encounter:
The image shows a web browser displaying a Jinja2 "TemplateNotFound" error for "dash.html" with a traceback of the error. On the left, there's a file explorer showing project files in a code editor.
jinja2.exceptions.TemplateNotFound: dash.html
127.0.0.1 - - [20/Mar/2025 21:37:51] "GET /dashboard HTTP/1.1" 500 -

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The Faulty Route

In app.py, the dashboard route is defined as: 
@app.route('/dashboard')
@login_required
def dashboard():
    db = get_db()
    # ... build query ...
    return render_template('dash.html', tasks=tasks, users=users, statuses=statuses)

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Feeding the Error and Code to Composer

jinja2.exceptions.TemplateNotFound: dash.html

# app.py (excerpt)
@app.route('/dashboard')
@login_required
def dashboard():
    db = get_db()
    # Build the SELECT query...
    return render_template('dash.html', tasks=tasks, users=users, statuses=statuses)
Ask Composer:
“Help me debug this error.”
Composer identifies:
Issue: You reference dash.html, but your templates directory contains dashboard.html.
Solution: Rename the template or update the call to render_template('dashboard.html', …).
Always verify your templates/ folder matches the filenames you pass to render_template().

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Applying the Fix

-    return render_template('dash.html', tasks=tasks, users=users, statuses=statuses)
+    return render_template('dashboard.html', tasks=tasks, users=users, statuses=statuses)
Reload the page—the dashboard now renders correctly.

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2. Debugging a Database Column Typo

Next, attempt to create a new task via the web interface:
The image shows a "Task Manager" web application interface where a user can create a new task by entering details like title, description, assignee, and status. The interface also includes options to filter tasks by status and assignee.
Submitting the form triggers: 
sqlite3.OperationalError: table tasks has no column named statu3

Traceback (most recent call last):
  File ".../flask/app.py", line 1478, in __call__
    response = self.handle_exception(e)
  ...
  File "app.py", line 193, in create_task
    db.execute(...)

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The Faulty INSERT Statement

In app.py, the handler is: 
@app.route('/task/create', methods=['GET', 'POST'])
@login_required
def create_task():
    if request.method == 'POST':
        title = request.form['title']
        description = request.form['description']
        status = request.form['status']
        assigned_to = request.form['assignee']

        db.execute(
            'INSERT INTO tasks (title, description, statu3, created_by, assigned_to) '
            'VALUES (?, ?, ?, ?, ?)',
            (title, description, status, session['user_id'], assigned_to)
        )
        db.commit()
        return redirect(url_for('dashboard'))
    return render_template('create_task.html')

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Feeding Code and Error to Composer

sqlite3.OperationalError: table tasks has no column named statu3

db.execute(
    'INSERT INTO tasks (title, description, statu3, created_by, assigned_to) VALUES (?, ?, ?, ?, ?)',
    (title, description, status, session['user_id'], assigned_to)
)
Composer responds:
Cause: The column name statu3 is mistyped. It should be status.
Fix: Correct the column name in the SQL statement.
Double-check your database schema (e.g., schema.sql) before running migrations or inserts.

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Applying the Fix

-    'INSERT INTO tasks (title, description, statu3, created_by, assigned_to) VALUES (?, ?, ?, ?, ?)',
+    'INSERT INTO tasks (title, description, status, created_by, assigned_to) VALUES (?, ?, ?, ?, ?)',
Save and refresh—the new task is created successfully.

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3. Best Practices for AI-Assisted Debugging

Error TypeCommon CauseRecommended Fix
Jinja2 TemplateNotFoundFilename mismatchUpdate render_template() to the correct name
sqlite3.OperationalErrorColumn name typoCorrect the column in your SQL statement
  1. Capture Complete Context
    • Include full tracebacks or error messages.
    • Paste only the relevant code where the error occurred.
  2. Provide Schema or Definitions
    • Share your schema.sql or data models so the AI can verify table and column names.
  3. Trust but Verify
    • Apply your own programming knowledge to confirm suggested changes align with your codebase.
  4. Experiment with Multiple Models
    • If Composer’s recommendation isn’t clear, try another model (e.g., GPT-3.5, GPT-4, Gemini) or start a fresh session.
  5. Use Logs as Supplemental Context
    • Include application or server logs (from SSH sessions or production environments) for deeper insights.

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