Note (2018-04-09): Slack's API has changed since I wrote this article. I also never wrote a part 2. If you want an up to date tutorial this blog from DigitalOcean is good.

Part 1: Setting Up Our Workflow and a Simple Application

A few weekends ago my pet project was to set up a drive time slash command in Slack. Searching through our organization’s Slack conversation history, on top of overhearing several conversations, it seems like traffic is both a source of anguish and a favorite topic for smalltalk in our office.

With that in mind, I set out to create a Slash Command for our Slack team. The end product was a slash command command that returned real time traffic and drive time data from the Google Directions API to Slack.

The process was a bit arduous, especially for a relative web development newbie. There’s quite a bit to understand for all the pieces to come together, so I thought I would document a the process to help out other Slackers looking to create custom Slack commands. In addition, I’ll walk through some of the debugging and problem solving I did while creating an app. We won’t be replicating the /drive command, but be working with something more simple for the purposes of this example.

Final Product for the Complete Tutorial

When we’re done with the complete tutorial we will have created a slash command for Slack that allows a user to get information on expected rainfall rain at a specified location. Our final command will be /rain, but we’ll be making a simple /hello command for this part of the tutorial.



  • Slack
  • Python 3.x (It will probably work on 2.x as well)
  • Flask
  • A text editor (I used Sublime Text 2, but am starting to love Atom)
  • ngrok for testing our application
  • API Key


  • Basic command line familiarity
  • Basic python skills (package installation, basic syntax)

Other Notes: I built this on OSX, so some language or operations may be OS specific.

Setting up a Workflow

Let’s create a simple “Hello World” Flask app to understand the foundational structure of a Flask application. As we do so, we’ll walk through the steps of a workflow that will allow us to test our application from within Slack.

Creating a Flask App

Create a new folder for your application and inside of it create a new python file – will work. Insert the following code into that file:

from flask import Flask
import os

app = Flask(__name__)

@app.route('/hello', methods=['POST'])
def hello():
    return 'Hello Slack!'

if __name__ == '__main__':
    port = int(os.environ.get('PORT', 5000))'', port=port, debug=True)

This is actually our entire application. Let’s take a look at one block at a time to understand the code.

Importing Flask & Creating an Application Object

from flask import Flask
import os

app = Flask(__name__)

In this step we’re importing the required libraries. We’re importing the Flask class from the flask package, as well as the os package, which we’ll need later in our code.

We’re also creating an application object named app from the Flask class. Don’t worry about the __name__ parameter for the Flask object, if you’re really curious here’s some detail.

Defining Routing and Request Methods

@app.route('/hello', methods=['POST'])
def hello():
    return 'Hello Slack!'

This is the functional meat of our application so let’s break it down. @app.route() is a function decorator – it basically modifies something about the next function we’re going to declare. The first parameter to this decorator is '/hello'. This parameter is critically important: it defines what happens when someone goes to the URL The second parameter is methods=['POST']. This is telling us that we are only going to accept POST requests at this route.

The idea of requests were a bit perplexing to me at first so I’ll try and briefly explain. At the most basic level, communication on the internet mostly exists between clients and servers (there’s actually a good Simple Wikipedia article on this). For example: when you’re surfing the web, you’re actually making a GET request as a client to the server in order to view a webpage. This request tells a website or application to “give the client some information”. For this current page that information is HTML and some other files that are then interpreted by your web browser and displayed. POST requests tell a site or application that it will be receiving information at that URL. That is: instead of a website or application sending something, the application will receive something. In our case, our application is going to receive the POST request from Slack, and that request will contain some information which our application can use.

Finally we define the hello() function, which defines what happens when someone uses our /hello route. In this case, all we’re going to do is return a string to them that says Hello Slack! if a request is made.

Declaring the Application Port

if __name__ == '__main__':
    port = int(os.environ.get('PORT', 5000))'', port=port, debug=True)

This last chunk does something simple: when you run this program (and it’s the main program), find the port of 5000, and then finally serve the application locally using that port.

Testing our Application with ngrok

Our application should be ready to test. Open terminal and navigate to your project folder, and type python to start our program. Terminal should pop up some information about where the server is running and that the debugger is active. Now we have to test our application.

This was a step that actually stumped me for a while. Previously, I had built applications or websites that only accepted GET requests. In this case, we can test it locally by just running the python file we created and browsing to http://localhost:5000/hello and we can see our application. If we did that now with our application, we’ll get a nice Method Not Allowed page since our application only accepts post requests.

We could send POST requests from our computer to our application locally using curl, but that’s more work than just loading a page in a browser and, more importantly, we would have to put some effort into making sure our request was in the same format as it would come from Slack. Additionally, our request won’t be coming from our local computer to our local server, it will be coming from wherever the Slack server is to our server. Because of that, we can’t use localhost for testing with Slack, since localhost (relative to Slack, which is sending the request) is where the Slack server is, and it doesn’t have our application files.

Luckily, there’s a fantastic application called ngrok that allows us to create a public URL to access a local server. We can use this to host our application locally, then point our Slack application to the public URL generated by ngrok. Using ngrok is fairly simple: download the file, open a shell to where you downloaded the file (or take this time to add it to your path), and type ngrok http 5000 in the shell. You should now see something like this in your terminal:

ngrok by @inconshreveable                                       

Tunnel Status                 online
Version                       2.0.19/2.0.19
Web Interface       
Forwarding           -> localhost:5000
Forwarding           -> localhost:5000

Connections                   ttl     opn     rt1     rt5     p50     p90
                            0       0       0.00    0.00    0.00    0.00

If we were accepting GET requests in our app, we could now head to the URLs above and view our page. However, if we go there now, nothing will happen since we’re only accepting POST requests.

Adding our Slash Command to Slack

To add a slash command integration to your Slack channel, head to, then navigate to Browse apps > Custom Integrations > Slash Commands and click the Add Configuration button.

Our slash command for our example will start as /hello. Type that in and click Add Slash Command Configuration.

After then, we have some options to configure on our app. The URL field is critical here. We’re going to paste the URL that ngrok gave us, being sure to append /hello at the end. That means our URL should be something similar to: The integration settings should match below:

Displaying the Response


Now we can save our integration, head to a Slack channel (I prefer DMs with slackbot), and type /hello into the chat. We should see a response of “Hello Slack!” immediately after.

Displaying the Response

You should also notice the requests popping up in the ngrok Terminal window every time you issue the command.

In Summary

Using Python, Flask, and ngrok we developed and tested a simple Flask application that responds to the /hello command in Slack.

Some key ideas that took me a while to comprehend:

  • URLs accept multiple kinds of requests: GET and POST are the most common
    • GET says: “give me some information”
    • POST says: “I’m giving you information”
  • We can’t test complex POST requests easily using localhost. Fortunately, we can use ngrok to get a public URL for a server we’re running locally and send it POST requests from Slack

Next Time

In the next part (Creating Slack Slash Commands with Python and Flask: Part 2: Electric Bugaloo), we’ll extend our app to use a weather API that returns some information about expected rainfall. We’ll also work to better understand how Slack sends requests to our app and what information is contained in a POST request.