Getting Started with Flask: A Beginner’s Guide to Python Web Development

Certainly! Here’s a detailed step-by-step guide titled "Getting Started with Flask: A Beginner’s Guide to Python Web Development" to help you set up and fix common issues while starting your Flask journey.

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Flask is a lightweight and flexible web framework for Python that is perfect for beginners. This guide will walk you through installing Flask, setting up a basic web app, and troubleshooting common errors.

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Step 1: Install Python

Why?

Flask is a Python framework, so you need Python installed on your machine.

How?

• Check if Python is installed:
  Open your terminal (Command Prompt, Powershell, Terminal):
  bash
  python –version

  or
  bash
  python3 –version

  If you see a version number (e.g., Python 3.9.7), Python is installed.

• If Python is not installed:

  • Download from python.org
  • Follow the installation instructions.
  • Ensure you check the box "Add Python to PATH" during installation (important for using Python from the command line).

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Step 2: Create a Virtual Environment (Recommended)

Why?

A virtual environment isolates your project and its dependencies from global Python installations.

How?

• Create a folder for your project:
  bash
  mkdir flask_project
  cd flask_project

• Create a virtual environment:
  bash
  python -m venv venv

  or, if your system uses python3:
  bash
  python3 -m venv venv

• Activate the virtual environment:

  • On Windows (Command Prompt):

  venv\Scripts\activate

  • On macOS/Linux:

  source venv/bin/activate

• You should now see (venv) at the start of your terminal prompt indicating the environment is active.

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Step 3: Install Flask

Why?

You need to install the Flask package inside your virtual environment.

How?

• Run:
  bash
  pip install Flask

Troubleshooting:

• If pip is not recognized:

  • Ensure your virtual environment is activated.
  • Use python -m pip install Flask as an alternative.

• If you get permissions errors, make sure you are not using sudo or running outside a virtual environment.

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Step 4: Create Your First Flask Application

How?

• Inside your project folder, create a new Python file, e.g., app.py.
• Open app.py in a text editor and paste the following starter code:

python
from flask import Flask

app = Flask(name)

@app.route(‘/’)
def home():
return "Hello, Flask!"

if name == "main":
app.run(debug=True)

• Explanation:

  • Flask(__name__) initializes the app.
  • @app.route('/') creates a route for the homepage.
  • app.run(debug=True) starts the server with debug mode on (helpful for catching errors).

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Step 5: Run Your Flask Application

How?

• Make sure your virtual environment is activated.

• Run the file:
  bash
  python app.py

  or if your system requires:
  bash
  python3 app.py

• You should see output like:

  • Running on http://127.0.0.1:5000/ (Press CTRL+C to quit)
  • Restarting with stat
  • Debugger is active!
  • Debugger PIN: 123-456-789

• Open your web browser and visit http://127.0.0.1:5000/.
• You should see the text: Hello, Flask!

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Step 6: Common Errors & How to Fix Them

1. ModuleNotFoundError: No module named ‘flask’

• Cause: Flask is not installed in your current environment.
• Fix:

  • Ensure virtual environment is activated ((venv) prefix).
  • Run pip install Flask.
  • Verify installation with pip list and look for Flask.

2. ‘python’ is not recognized as an internal or external command

• Cause: Python is not added to your PATH environment variable.
• Fix:

  • Reinstall Python and check "Add Python to PATH" option.
  • Alternatively, use full Python path or edit the Path variable manually.

3. Port 5000 already in use

• Sometimes another app is using Flask’s default port.

• Fix:

  • Run Flask on a different port, e.g.:
    bash
    python app.py –port=5001

  Or edit app.run():
  python
  app.run(debug=True, port=5001)

4. IndentationError or SyntaxError

• Python is sensitive to indentation.
• Make sure your code is properly indented (4 spaces per indentation level).
• Avoid mixing tabs and spaces.

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Step 7: Next Steps

• Start building more routes and templates.
• Learn how to:

  • Use HTML templates with Jinja2 (render_template()).
  • Handle form submissions.
  • Connect to a database.
  • Organize your Flask project structure.

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Additional Tips

• Always activate the virtual environment when working on your project.
• Use pip freeze > requirements.txt to save dependencies.
• Share your requirements.txt so others can install dependencies with:
  bash
  pip install -r requirements.txt

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If you encounter any specific issues, feel free to share the error message or describe the problem, and we can troubleshoot further!