Working with data files

Data files are extremely powerful ways to test your APIs with varying data to check if they behave properly under unexpected circumstances.

We can think of data files are parameters for each iteration of a collection run. Let’s walk through an example.

Download the collection and data files used in this example:

post request

Here, we have a simple collection with a single POST request. If you open up this request, you’ll see two variables used in the request, path (in the URL) & value (in the request body). These are used just like environment variables. We will supply the value to these variables using a JSON / CSV file. On opening the test script, you’ll see we’re using some variables in the test script -data specifically. This isn’t defined in the script itself. The Postman Sandbox initializes the data variable from the JSON/CSV file that we will select in the collection run.

using the data variable

Let’s investigate the data files first. We currently support JSON & CSV files.

The JSON data file looks like this:

      "path": "post",
      "value": "1"
    }, {
      "path": "post",
      "value": "2"
    }, {
      "path": "post",
      "value": "3"
    }, {
      "path": "post",
      "value": "4"

This is an array of objects. Each object represents the variable values for one iteration. Each member of this object represents a variable. In this way, in the first iteration, the variable called path will have the value post, and the variable value will have the value 1. Similarly, in the second iteration, path will still be post and value will be 2. In this example, the variable path does not change it’s value over iterations, but value does. This is totally up to you.

The data file can also be a CSV. The example CSV looks like this:

    path, value
    post, 1
    post, 2
    post, 3
    post, 4

In typical CSV fashion, the first row represents all variable names, and subsequent rows represent values for these variables for each iteration. For iteration 1, path has value post, and value is 1. For the second iteration, path is still post, but value is 1.

Do note that you can only use one data file for one run.

Now that you understand how to construct data files, let’s supply this data file to a Collection Run. Click Select File in the Runner, and select one of these files. You can also preview what values each variable has in each iteration by clicking on Preview next to the file name.

collection runner view             

preview data

Let’s run our collection now. You’ll see that all tests pass now. If you open up the request debug tooltip, and expand Request Body, you’ll see that the variable {{value}} was replaced by the value, as dictated by the data file. Read more about debugging requests. In fact, for different iterations, this value is different. This way, we’ve thrown different kinds of data to our API and have ensured that it works correctly for each case.

request debug tooltip

Let’s also take a look at our test scripts once again. The variable data is a predefined variable that gets the values from the data file. With each iteration, it’s value is updated with new data from our file. data is an object with all variables you defined in your file as it’s keys. Since this API echoes back whatever is sent to it, we’re asserting that the returned value from Echo is the same as the one dictated by our file.

Data variables can be used in all places that environment variables can be used, in the exact same way, except in pre-request & test scripts.