Custom error pages in Flask

• Posted 26 January 2024
• Tagged with python

You can customise the error pages by adding a few instances of app.error_handler, for example:

@app.errorhandler(404)
def not_found(e: Exception) -> str:
    return render_template("not_found.html"), 404


@app.errorhandler(500)
def internal_server_error(e: Exception) -> str:
    return render_template("internal_server_error.html"), 500

Some notes:

• I was a bit concerned that the exception might get swallowed here, but it’s all good – when you throw a 500 error, the underlying exception will still get logged, including the full stack trace.

• If for some reason the 500 page fails to render correctly, Flask falls back to a generic “Internal Server Error” page.

• I find it helpful to add a route that I can use to test the behaviour of 500 pages in the prod environment, which looks like this:

  @app.route("/500")
  def deliberate_error() -> str:
      raise ValueError("BOOM!")