Get referring URL for Flask request

request.referrer contains the URL the request came from, although it might not be sent by the client for various reasons.

The attribute takes its value from the Referer (not a typo!) header:

referrer = request.headers.get("Referer")

or, using the Flask shortcut:

referrer = request.referrer

See this tutorial for an example.

Thanks to the accepted answer, I set up my app to capture an external referrer and store it in the session. Then when the user signs up I save that value with the user.

from flask import request, g
from werkzeug.urls import url_parse

def referral():
    url = request.referrer

    # if domain is not mine, save it in the session
    if url and url_parse(url).host != "":
        session["url"] = url

    return session.get("url")

def before_request():
    g.user = current_user
    g.url = referral()