Develop Reference

How to create a paypal payment with a custom amount?

I would like to integrate python's paypal sdk into my project so that users can pay however much they choose. The current method requires a fixed price, https://developer.paypal.com/docs/api/quickstart/create-process-order/
However I want the user to be able to choose how much they want to send.

You linked to a server integration document, which is also for a deprecated API.
Do you want to create the order on the server side, or not? If yes, sending the amount from the client to the server that executes the orders API call is work you will need to do.
Begin your integration with the current v2/checkout/orders API by making two routes on your server, one for 'Create Order' and one for 'Capture Order', documented here. These routes should return only JSON data (no HTML or text). When a capture response is successful, store its resulting payment details in your database (particularly purchase_units[0].payments.captures[0].id, the PayPal transaction ID) and perform any necessary business logic (such as sending confirmation emails or reserving product) before sending your return JSON.
Pair those two routes with the following approval flow: https://developer.paypal.com/demo/checkout/#/pattern/server
Since your question was specific to setting a custom amount, you should probably add a JSON body to the createOrder's fetch call, to send the amount you want to your server.
For a simple client-side integration that uses no server-side API calls, see the documentation and
set the createOrder purchase units value: with some function or variable that gets the value you want. For instance:
value: document.getElementById(...).value
If you are not a programmer, you can even generate such a button for a custom amount via https://www.paypal.com/buttons/smart

Way to get timestamp of last activity for all Slack users

I am building a Python script which retrieves a set of information for all Slack users of the organization that I work. Currently, I was asked by the stakeholders to retrieve the last time that our organization's users were active on Slack. I came across the users.getPresence method but this can only return a timestamp for the owner of the token. Has anybody worked on that before? Thanks.

It depends a bit how you define "active on Slack".
If getting the date and time a user was logged in last is enough, take a look at team.accessLogs, which will give you the timestamp of the last login for every user (date_last).
This will not reflect whether the user performed any activity, e.g. posting a message or uploading a file though.
However, note that this API method works for paid plans only.

How can I use django to realize real time?

I have a rethinkdb. Data will get in database for every five minutes.
I want to create a website to real-time inspect this data flow from rethinkdb.
That is, when surfing the webpage, the data from db on webpages can update automatically without refreshing the webpage.
I know there are several ways to make it real-time such as django channels or websockets. However, model in django does not support rethinkdb.
Sorry I am a layman of making website and may express things inaccurately.
Can someone give me a keyword or hint?

If you make your question more specific, the community here will be able to offer you better support.
However, here is a general solution to your problem.
You will need to do two things:
Create a backend API that allows you to:
Check if new data has been added to the database
Fetch new data via a REST api request
Make frontend AJAX requests to this api
Fetch data
Periodically (every 30sec) check if there is new data
Fetch data again if new data is detected
To do this using Django as the backend, I would recommend using the Django Rest Framework to create your API.
This API should have two endpoints:
ListView of your data
Endpoint returning the id and timestamp of the last datapoint
Next you will have to create a frontend that uses javascript to make requests to these endpoints. When you fetch data, store the id and timestamp of the most recent data point. Use this to check if there is new data.
I would recommend using a Javascript framework such as Angular or react but depending on your needs these may be overkill.
EDIT:
Now that you have updated your answer to be more specific, here is my advice. It sounds like your number one priority is rethinkDB and real time data. Django is not well suited this because it is not compatible with rethinkDB. Real time support has come a long way in Django with Django channels however.
It sounds like you are early on in your project and have little to no codebase in Django. I would recommend using horizon along with rethink db. Horizon is a javascript backend built for real time data from rethinkdb.

How the user of my Django app can create "customized" tasks?

I am new to Celery and I can't figure out at all how I can do what I need. I have seen how to create tasks by myself and change Django's file settings.py in order schedule it. But I want to allow users to create "customized" tasks.
I have a Django application that is supposed to give the user the opportunity to create the products they need (files that gather weather information) and to send them emails at a frequency they choose.
I have a database that contains the parameters of every product like geographical time, other parameters and frequency. I also have a script sendproduct.py that sends the product to the user.
Now, how can I simply create for each product, user created a task that execute my script with the given parameters and the given frequency?

As you know how to create & execute tasks, it's very easy to allow customers to create tasks.
You can create a simple form to get required information from the user. You can also have a profile page or some page where you can show user preferences.
The above form helps to get data(like how frequently he needs to receive eamils) from user. Once the form is submitted, you can trigger a task asynchronously which will does processing & send emails to customer.

Get list of connected users with Django

I'm looking for a way to keep track of users that are online/offline. So if I present all users in a list i could have an icon or some kind of flag to show this. Is this built in in Django's default Auth system?
My first thought was to simply have a field in my profiles called last_logout in the models and update it with the date/time each time user logged out.
With this info and the built in last_login I should be able to make some kind of function to determine if the user is loggedin/online right?
Or should I just have a boolean field called "online" that I can change when user logs in and out?

With only django it will be hard to do. For such task async frameworks are more suitable.
For example, tornado.
Users won't do logout explicitly every time they go offline. They just close their browser and that's it. You can't know it with only django auth app. It is not designed for such tasks.
Even if you will check for not expired session, it not gives you all online users, because session can be non-expired for 30 days.
So to get real online users, possible solutions are:
Every user will send some data via javascript to your server, for example every 10 seconds. You can fetch that data on server and put user into cache and set cache key to be alive for 10 seconds. So when you need to know, who are online now, you'll check your cache. But it is not a good solution, because it will need a lot of server resources.
Use async framework (tornado) at server side (you can setup separate process for exact requests). And use websockets (SockJS is a good library for that at client side). It is a more complicated solution, but it is better.

You have to consider what exactly means for the users to be "online". Since any user can close the browser window any time and without the server knowing about that action, you'd end up having lots of false "online" users.
You have two basic options:
Keep track of the user's last activity time. Every time the user loads a page you'd update the value of the timer. To get a list of all online users you'd need to select the ones with an activity before X minutes. This is what is done by some web forums.
Open a websocket, long polling connection or some heartbeat to the server. This is what Facebook chat does. You'd need more than just django, since to keep a connection open another kind of server-side resources are needed.