Question

I’m trying to use flask.g to store variables that can be accessed in other functions, but I don’t seem to be doing something correctly. The application generates the following error when I try to access g.name: AttributeError: '_RequestGlobals' object has no attribute 'name'.

The documentation for flask.g says:

Just store on this whatever you want. For example a database
connection or the user that is currently logged in.

Here’s a complete, minimal example that illustrates the error that I receive when trying to access the variable outside of the function it was created in. Any help would be greatly appreciated.

#!/usr/bin/env python

from flask import Flask, render_template_string, request, redirect, url_for, g
from wtforms import Form, TextField

application = app = Flask('wsgi')

@app.route('/', methods=['GET', 'POST'])
def index():
    form = LoginForm(request.form)

    if request.method == 'POST' and form.validate():
        name =  form.name.data
        g.name = name

        # Need to create an instance of a class and access that in another route
        #g.api = CustomApi(name)

        return redirect(url_for('get_posts'))

    else:

        return render_template_string(template_form, form=form)

@app.route('/posts', methods=['GET'])
def get_posts():
    # Need to access the instance of CustomApi here
    #api = g.api
    name = g.name
    return render_template_string(name_template, name=name)


class LoginForm(Form):
    name = TextField('Name')

template_form = """
{% block content %}
<h1>Enter your name</h1>

<form method="POST" action="/">
    <div>{{ form.name.label }} {{ form.name() }}</div><br>
    <button type="submit" class="btn">Submit</button>    
</form>
{% endblock %}

"""

name_template = """
{% block content %}

    <div>"Hello {{ name }}"</div><br>

{% endblock %}

"""

if __name__ == '__main__':
    app.run(debug=True)

Answer 1

If you need to track authentication information, I’d suggest one of the Flask plugins like Flask-Login or Flask-Principal.

For example, we use Flask-Principal. It raises the identity-loaded signal when somebody authenticates (or it detects an authentication cookie). We then map their logged-in identity with a user in our database. Something like this:

# not actual code
@identity_loaded.connect_via(app)
def on_identity_loaded(sender, identity):
  user = Person.query.filter(Person.username==identity.person.username).one()
  g.user = user

and then we can use g.user in any controller or template. (We’re actually ripping a lot of this out, it was a easy, lazy hack that’s caused more trouble than it’s worth.)

If you don’t want to use a module, there’s a built-in signal you can hook into at the start of every request:

http://flask.pocoo.org/docs/tutorial/dbcon/

# This runs before every request
@app.before_request
def before_request():
    g.user = your_magic_user_function()

and g.user would then be magically available everywhere.

I hope that helps!