Error msg to user in django - html

Question

I have an html page where user insert data and click on ok button.
When button click a python function is checking if everything is ok and in that case update my DB.
In one of the cases when a check is false I want to pop up a msg to the user letting him know the operation faild.
What I did is:
if a<b:
return render(request, 'main/error.html')
and in error.html I puted the error details.
However this is extrimly inconvinet as the page is changing to error.html.
I want to give this error msg as Messege box or something like that... so that it stays on the same page.
The thing is that from views.py I can't access the html page and ask it to pop a msg.
How can I pop up a msg when a condition is false?

Answer 1

You can use the messaging framework in order to display a message on your page:
Quite commonly in web applications, you need to display a one-time notification message (also known as “flash message”) to the user after processing a form or some other types of user input.
For this, Django provides full support for cookie- and session-based messaging, for both anonymous and authenticated users. The messages framework allows you to temporarily store messages in one request and retrieve them for display in a subsequent request (usually the next one). Every message is tagged with a specific level that determines its priority (e.g., info, warning, or error).
There is extensive documentation and examples on the link provided.
EDIT: Example
Summary of settings (settings.py):
INSTALLED_APPS = (
...
'django.contrib.sessions',
'django.contrib.messages',
...
)
MIDDLEWARE_CLASSES = (
...
'django.contrib.sessions.middleware.SessionMiddleware',
...
'django.contrib.messages.middleware.MessageMiddleware',
...
)
TEMPLATES = [
{
'BACKEND': 'django.template.backends.django.DjangoTemplates',
'DIRS': [
os.path.join(BASE_DIR, 'templates'),
],
'APP_DIRS': True,
'OPTIONS': {
'context_processors': [
...
'django.contrib.messages.context_processors.messages',
],
'debug': True,
},
},
]
In a view:
from django.contrib import messages
if a < b:
messages.warning("a is less than b.")
In a template (example using bootstrap3):
{% if messages %}
<div class="messages">
{% for message in messages %}
<div class="alert alert-{{ message.tags }} alert-dismissible" role="alert">
<button type="button" class="close" data-dismiss="alert" aria-label="Close">
<span aria-hidden="true">×</span>
</button>
{{ message }}
</div>
{% endfor %}
</div>
{% endif %}