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Flask error handler that renders a custom error page when a 404 (Page Not Found) error occurs.

Purpose

This function serves as a Flask error handler specifically for HTTP 404 errors. When a user attempts to access a non-existent route in the Flask application, this handler intercepts the error and returns a user-friendly error page instead of the default Flask 404 page. It provides a consistent error experience across the application by using a custom template.

Source Code

def not_found_error(error):
    return render_template('error.html', error="Page not found"), 404

Parameters

Parameter Details

error: The error object automatically passed by Flask's error handling mechanism when a 404 error occurs. Contains information about the error that triggered this handler, though it is not used in the function body.

Return Value

Returns a tuple containing two elements: (1) the rendered HTML template 'error.html' with an error message 'Page not found' passed as a template variable, and (2) the HTTP status code 404. Flask uses this tuple to construct the HTTP response sent to the client.

Dependencies

• flask

Required Imports

from flask import render_template

Usage Example

from flask import Flask, render_template

app = Flask(__name__)

@app.errorhandler(404)
def not_found_error(error):
    return render_template('error.html', error="Page not found"), 404

# When a user navigates to a non-existent route like '/nonexistent'
# Flask will automatically call this handler and display the error page

if __name__ == '__main__':
    app.run()

Best Practices

• Ensure the 'error.html' template exists and is properly designed to handle error messages
• Consider logging the 404 error for monitoring purposes before returning the response
• The error parameter could be used to extract additional context about the failed request if needed
• Maintain consistent error handling patterns across all error handlers (404, 500, etc.)
• Test that the error handler works correctly by attempting to access non-existent routes
• Consider adding user-friendly navigation links in the error template to help users return to valid pages

Tags

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