Django 1.10+

Use an attribute, not a method:

if request.user.is_authenticated: # <-  no parentheses any more!
    # do something if the user is authenticated

The use of the method of the same name is deprecated in Django 2.0, and is no longer mentioned in the Django documentation.

Note that for Django 1.10 and 1.11, the value of the property is a CallableBool and not a boolean, which can cause some strange bugs.
For example, I had a view that returned JSON

return HttpResponse(json.dumps({
    "is_authenticated": request.user.is_authenticated()
}), content_type='application/json') 

that after updated to the property request.user.is_authenticated was throwing the exception TypeError: Object of type 'CallableBool' is not JSON serializable. The solution was to use JsonResponse, which could handle the CallableBool object properly when serializing:

return JsonResponse({
    "is_authenticated": request.user.is_authenticated
})

Following block should work:

    {% if user.is_authenticated %}
        <p>Welcome {{ user.username }} !!!</p>       
    {% endif %}

In your view:

{% if user.is_authenticated %}
<p>{{ user }}</p>
{% endif %}

In you controller functions add decorator:

from django.contrib.auth.decorators import login_required
@login_required
def privateFunction(request):

If you want to check for authenticated users in your template then:

{% if user.is_authenticated %}
    <p>Authenticated user</p>
{% else %}
    <!-- Do something which you want to do with unauthenticated user -->
{% endif %}

to check if user is logged-in (authenticated user) in views.py file, use "is_authenticated" method, as the following example:

def login(request):
    if request.user.is_authenticated:
        print('yes the user is logged-in')
    else:
        print('no the user is not logged-in')

to check if user is logged-in (authenticated user) in your html templates file you can use it also as the following example :

 {% if user.is_authenticated %}
    Welcome,{{request.user.first_name}}           

 {% endif %}

this is just example , and change it based on your requirements.

i hope this helpful for you .

For Django 2.0+ versions use:

    if request.auth:
       # Only for authenticated users.

For more info visit https://www.django-rest-framework.org/api-guide/requests/#auth

request.user.is_authenticated() has been removed in Django 2.0+ versions.