Running the Dev Server

During development you typically run:

• flask run

This starts Werkzeug’s development server.

Typical output

You’ll usually see:

• the URL (like http://127.0.0.1:5000)
• that it’s a development server

Host and port

• Default host: 127.0.0.1 (only accessible from your machine)
• Default port: 5000

You can change them:

bash

flask run --host 0.0.0.0 --port 8000

When should you use 0.0.0.0?

Use it when you want access from another device on your network.

Be careful: exposing dev servers is not recommended on public networks.

What happens when you visit the page?

1. Browser connects to the host/port
2. Sends GET /
3. Dev server forwards to Flask app
4. Flask finds a matching route and runs your view function
5. Response is returned

If you get a 404, it usually means:

• you didn’t define the route, or
• your URL doesn’t match the route rule

What the routing table looks like before any request

Flask builds a url_map at import time. Every rule you registered is in it — plus one you did not write:

routes.py

for rule in app.url_map.iter_rules():
    print(rule, rule.endpoint, sorted(rule.methods - {"HEAD", "OPTIONS"}))

measured output

/                       index    ['GET']
/api/items              items    ['GET', 'POST']
/static/<path:filename> static   ['GET']     <- added automatically
/user/<name>            user     ['GET']

HEAD and OPTIONS are attached to every GET rule for you, and a /static/ route exists from the moment the app object is created.

How a request finds a view

diagram Diagram mermaid

  flowchart TD
  R["incoming request"] --> M{"does a rule match the path?"}
  M -->|"no"| E404["404 Not Found"]
  M -->|"yes, but wrong method"| E405["405 Method Not Allowed
with an Allow header"]
  M -->|"yes"| V["run the view function"]
  V --> OK["200 and your return value"]
  V -->|"raises"| E500["500, or the debugger if debug is on"]

Measured against the app above:

request	status	note
GET /nope	404	no rule matches
DELETE /api/items	405	rule exists, method does not
Allow header on that 405		GET, POST, OPTIONS, HEAD

A 405 rather than a 404 is a useful signal: the URL is right and the verb is wrong.

The trailing slash rule is asymmetric

slashes.py

@app.route("/dir/")     # declared WITH a slash
def d(): ...
 
@app.route("/file")     # declared WITHOUT one
def f(): ...

declared	requested	result
/dir/	/dir	308 redirect to /dir/
/file	/file/	404

A rule ending in a slash behaves like a directory: ask without the slash and Flask redirects you. A rule without one behaves like a file: the slashed form simply does not exist. The redirect costs an extra round trip, and a POST that gets redirected can lose its body in some clients — so declare the form you intend and link to it exactly.

The dev server is not a production server

Werkzeug prints this on every start: “This is a development server. Do not use it in a production deployment.” It is single-process by default, does not handle load, and has no hardening. Deploy behind gunicorn, uwsgi or waitress.

See it move

sketch Matching a request against the url_map p5.js Pause Restart

Flask checks the path first, then the method. That order is why a wrong verb gives 405 rather than 404.

Check yourself

pch.quizTag pch.quizDefaultTitle

1. A route is declared as @app.route('/dir/') and a client requests /dir. What happens?

   A 404, because the paths differ B 200, the view runs directly C 308 redirect to /dir/ D 405 Method Not Allowed
   pch.quizShowAnswer

   C — 308 redirect to /dir/ — A rule ending in a slash behaves like a directory and Flask redirects. The reverse is not symmetric: a rule declared '/file' requested as '/file/' gives 404.

2. DELETE /api/items returns 405 rather than 404. What does that distinction tell you?

   A the server is misconfigured B the path matched a rule but the method is not allowed for it C the view raised an exception D the route requires authentication
   pch.quizShowAnswer

   B — the path matched a rule but the method is not allowed for it — Flask matches the path first, then the method. A 405 means the URL is right and the verb is wrong, and the response carries an Allow header listing what is permitted.

3. Which rule appears in app.url_map without you writing it?

   A a catch-all 404 handler B /static/<path:filename> C /favicon.ico D /health
   pch.quizShowAnswer

   B — /static/<path:filename> — Creating the Flask object registers the static route. Iterating url_map also shows HEAD and OPTIONS attached automatically to every GET rule.