python flask how to get route id from url

It's pretty straightforward - pass the path parameter in between angle brackets, but be sure to pass that name to your method.

@app.route('/page/<page_id>')
def page(page_id):
    pageid = page_id
    # You might want to return some sort of response...

You should use the following syntax:

@app.route('/page/<int:page_id>')
def page(page_id):
    # Do something with page_id
    pass