Redirects and Errors

Redirects and errors are part of normal web flow.

diagram the trailing slash rule, which is not symmetric mermaid

  flowchart TD
  A["@app.route('/dir/')"] --> B["GET /dir/"]
  B --> C["200"]
  A --> D["GET /dir"]
  D --> E["308 redirect to /dir/"]
  E --> F["the browser follows -- it works, at the cost of a round trip"]
  G["@app.route('/file')"] --> H["GET /file"]
  H --> I["200"]
  G --> J["GET /file/"]
  J --> K["404 -- no redirect, no forgiveness"]

Flask treats a rule ending in a slash as a directory and one without as a file, and the two behave differently when the request does not match. A directory rule forgives a missing slash with a redirect. A file rule does not forgive an extra one -- it returns 404. Both behaviours were measured.

Measured, both directions

Rule defined as	Request	Result
/dir/	GET /dir/	200
/dir/	GET /dir	308 redirect to /dir/
/file	GET /file	200
/file	GET /file/	404

The asymmetry is deliberate and matches how web servers have always treated directories and files, but it catches people because only one of the two mistakes is forgiven.

Two practical consequences. A 308 is an extra round trip on every request that omits the slash, so links should use the canonical form — url_for always does. And a POST to a URL that redirects can lose its body in some clients, which is why a form action pointing at the non-canonical spelling is worth fixing rather than relying on the redirect.

Redirects

A redirect tells the client: “go to a different URL.”

Common use cases:

• after successful form submission (POST → redirect → GET)
• redirecting legacy URLs

python

from flask import Flask, redirect, url_for
 
app = Flask(__name__)
 
 
@app.route("/old")
def old():
    return redirect(url_for("new"))
 
 
@app.route("/new")
def new():
    return "New page"

Why redirect after POST?

The PRG pattern (Post/Redirect/Get) prevents form re-submission if the user refreshes.

Errors

You can return an error status code directly:

python

@app.route("/forbidden")
def forbidden():
    return "No access", 403

Or use abort():

python

from flask import abort
 
@app.route("/admin")
def admin():
    abort(403)

abort() raises an HTTP exception that Flask turns into a proper response.

Common HTTP status codes

• 400 Bad Request — invalid input
• 401 Unauthorized — not logged in (auth missing)
• 403 Forbidden — logged in but not allowed
• 404 Not Found — route/resource doesn’t exist
• 500 Internal Server Error — unhandled exception

🧪 Try It Yourself

Exercise 1 – Create a Flask App

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Exercise 2 – Dynamic Route

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Exercise 3 – Return JSON

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