What does netloc mean?

Question

I'm learning to make login function with Flask-login, and I'm facing with this code in my tutorial that I'm following:

@app.route('/login', methods = ['GET', 'POST']) def login():     if current_user.is_authenticated:         return redirect(url_for('index'))     form = LoginForm()     if form.validate_on_submit():         user = User.query.filter_by(username=form.username.data).first()         if user is None or not user.check_password(form.password.data):             flash('Invalid username or password')             return redirect(url_for('login'))         login_user(user, remember=form.remember_me.data)         next_page = request.args.get('next')         if not next_page or url_parse(next_page).netloc != '': # what is it means in this line..?             next_page = url_for('index')         return redirect(next_page)     return render_template('login.html', title='Sign In', form=form) 

But I'm not sure what's the code above that I commented means..?, especially in netloc word, what is that..?, I know that is stand for network locality, but what is the purpose on that line..?

Answer 1

From RFC 1808, Section 2.1, every URL should follow a specific format:

<scheme>://<netloc>/<path>;<params>?<query>#<fragment> 

Lets break this format down syntactically:

• scheme: The protocol name, usually http/https
• netloc: Contains the network location - which includes the domain itself (and subdomain if present), the port number, along with an optional credentials in form of username:password. Together it may take form of username:[email protected]:80.
• path: Contains information on how the specified resource needs to be accessed.
• params: Element which adds fine tuning to path. (optional)
• query: Another element adding fine grained access to the path in consideration. (optional)
• fragment: Contains bits of information of the resource being accessed within the path. (optional)

Lets take a very simple example to understand the above clearly:

https://cat.com/list;meow?breed=siberian#pawsize 

In the above example:

• https is the scheme (first element of a URL)
• cat.com is the netloc (sits between the scheme and path)
• /list is the path (between the netloc and params)
• meow is the param (sits between path and query)
• breed=siberian is the query (between the fragment and params)
• pawsize is the fragment (last element of a URL)

This can be replicated programmatically using Python's urllib.parse.urlparse:

>>> import urllib.parse >>> url ='https://cat.com/list;meow?breed=siberian#pawsize' >>> urllib.parse.urlparse(url) ParseResult(scheme='https', netloc='cat.com', path='/list', params='meow', query='breed=siberian', fragment='pawsize') 

Now coming to your code, the if statement checks whether or not the next_page exists and whether the next_page has a netloc. In that login() function, checking if .netloc != '', means that it is checking whether the result of url_parse(next_page) is a relative url. A relative url has a path but no hostname (and thus no netloc). ;)