I would like to know how I can retrieve the list of people a user is following on Instagram. This is given that this particular user is someone that I follow. So I have access to his/her photos and his followers on the Instagram app.
How can I use the Instagram API to do this? Is this legal to do?
You can use the following Instagram API Endpoint to get a list of people a user is following.
https://api.instagram.com/v1/users/{user-id}/follows?access_token=ACCESS-TOKEN
Here's the complete documentation for that endpoint. GET /users/user-id/follows
And here's a sample response from executing that endpoint.
Since this endpoint required a user-id (and not user-name), depending on how you've written your API client, you might have to make a call to the /users/search endpoint with a username, and then get the user-id from the response and pass it on to the above /users/user-id/follows endpoint to get the list of followers.
IANAL, but considering it's documented in their API, and looking at the terms of use, I don't see how this wouldn't be legal to do.
Shiva's answer doesn't apply anymore. The API call "/users/{user-id}/follows" is not supported by Instagram for some time (it was disabled in 2016).
For a while you were able to get only your own followers/followings with "/users/self/follows" endpoint, but Instagram disabled that feature in April 2018 (with the Cambridge Analytica issue). You can read about it here.
As far as I know (at this moment) there isn't a service available (official or unofficial) where you can get the followers/followings of a user (even your own).
You can use Phantombuster. Instagram has set some rate limit, so you will have to use either multiple accounts or wait for 15 minutes for the next run.
来源：https://stackoverflow.com/questions/32407851/instagram-api-how-can-i-retrieve-the-list-of-people-a-user-is-following-on-ins