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

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