redirect while passing arguments

In flask, I can do this:

render_template("foo.html", messages={'main':'hello'})

And if foo.html contains {{ messages['main'] }}, the page will show hello. But what if there’s a route that leads to foo:

@app.route("/foo")
def do_foo():
    # do some logic here
    return render_template("foo.html")

In this case, the only way to get to foo.html, if I want that logic to happen anyway, is through a redirect:

@app.route("/baz")
def do_baz():
    if some_condition:
        return render_template("baz.html")
    else:
        return redirect("/foo", messages={"main":"Condition failed on page baz"}) 
        # above produces TypeError: redirect() got an unexpected keyword argument 'messages'

So, how can I get that messages variable to be passed to the foo route, so that I don’t have to just rewrite the same logic code that that route computes before loading it up?

Answer

You could pass the messages as explicit URL parameter (appropriately encoded), or store the messages into session (cookie) variable before redirecting and then get the variable before rendering the template. For example:

from flask import session, url_for

def do_baz():
    messages = json.dumps({"main":"Condition failed on page baz"})
    session['messages'] = messages
    return redirect(url_for('.do_foo', messages=messages))

@app.route('/foo')
def do_foo():
    messages = request.args['messages']  # counterpart for url_for()
    messages = session['messages']       # counterpart for session
    return render_template("foo.html", messages=json.loads(messages))

(encoding the session variable might not be necessary, flask may be handling it for you, but can’t recall the details)

Or you could probably just use Flask Message Flashing if you just need to show simple messages.