redirect while passing arguments

Question

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 1

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:

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.

Answer 2

I'm a little confused. "foo.html" is just the name of your template. There's no inherent relationship between the route name "foo" and the template name "foo.html".

To achieve the goal of not rewriting logic code for two different routes, I would just define a function and call that for both routes. I wouldn't use redirect because that actually redirects the client/browser which requires them to load two pages instead of one just to save you some coding time - which seems mean :-P

So maybe:

def super_cool_logic():
    # execute common code here

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

@app.route("/baz")
def do_baz():
    if some_condition:
        return render_template("baz.html")
    else:
        super_cool_logic()
        return render_template("foo.html", messages={"main":"Condition failed on page baz"})

I feel like I'm missing something though and there's a better way to achieve what you're trying to do (I'm not really sure what you're trying to do)