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# Building an OAuth 2 App

<Warning title="This guide assumes you've already registered an OAuth 2 app">
If not, please refer to [OAuth 2 Code Flow](/oauth-2-applications/oauth-2-code-authorization-flow) to configure your application.
</Warning>

## Application basics
**Note**: you should store your `client_id` and (if not using PKCE) `client_secret` safely and access them via your environment. The below examples assume the presence of a **.env** file containing the variables `CLIENT_ID` and `CLIENT_SECRET`.

**`Python`**

```python title="Python"
import urllib, requests, requests.auth
import os

CLIENT_ID = os.environ.get('CLIENT_ID')
CLIENT_SECRET = os.environ.get('CLIENT_SECRET')

AUTHORIZE_URL = "https://applications.frame.io/oauth2/auth"
TOKEN_URL = "https://applications.frame.io/oauth2/token"
# The scopes you've chosen for your app, space-delimited
SCOPE = "offline account.read asset.read" 
# The callback URI for your app
REDIRECT_URI = "https://yourapp.domain/callback"
```

## Invoking the auth server
First, your application will need to call the Frame.io auth server, which will then redirect the user to a login page.

**`Python`**

```python title="Python"
import uuid
from urllib.parse import urlencode

def create_auth_url():
 credentials = {
  'response_type': 'code',
  'redirect_uri': REDIRECT_URI,
  'client_id': CLIENT_ID,
  'scope': SCOPE,
  'state': str(uuid.uuid4())
 }
 url = (AUTHORIZE_URL + "?" + urlencode(credentials))
 return url
```

## The callback
The auth server will then make a `GET` request to your `REDIRECT_URI`, which in turn will need to call the `TOKEN_URL`. This callback will be slightly different depending on whether or not your application is configured to use PKCE.

### Without PKCE
If you're not using PKCE, your callback must include an `Authorization` header that includes your `CLIENT_ID` and `CLIENT_SECRET`. 

**`Python`**

```python title="Python"
def callback():
   state = request.args.get('state')
   scope = request.args.get('scope')
   code = request.args.get('code')
   error = request.args.get('error')

   if error:
     return "Error: " + error

   # Set up for client authorization and set up the data you need to send.
   client_auth = requests.auth.HTTPBasicAuth(CLIENT_ID, CLIENT_SECRET)

   post_data = {
     "grant_type": "authorization_code",
     "code": code,
     "redirect_uri": REDIRECT_URI,
     "state": state,
     "scope": SCOPE
   }

# Send a POST request with the data you need to receive an access token. 
# If everything goes well, it will be returned to you and you can use it with
# Frame.io.

   response = requests.post(TOKEN_URL, auth=client_auth, data=post_data)    
   return response.text
```

### With PKCE
If you're using PKCE, your callback must *not* include an `Authorization` header, but *must* include your `CLIENT_ID`  in its `POST` request body when calling back to the `TOKEN_URL`.

**`Python`**

```python title="Python"
def callback():
   state = request.args.get('state')
   scope = request.args.get('scope')
   code = request.args.get('code')
   error = request.args.get('error')

   if error:
     return "Error: " + error

# If using PKCE, you must include the CLIENT_ID in your request body  
   post_data = {
     "grant_type": "authorization_code",
     "code": code,
     "redirect_uri": REDIRECT_URI,
     "state": state,
     "scope": SCOPE
     "client_id": CLIENT_ID 
   }

# Send a POST request with the data you need to receive an access token. 
# If everything goes well, it will be returned to you and you can use it with
# Frame.io

  # If using PKCE, use the below request with no auth
  response = requests.post(TOKEN_URL, data=post_data) 

   return response.text
```

## Successful response
If your callback is successful, you will receive a JSON response that looks like this:
```json
{
  "access_token":"BEARER_TOKEN",
  "expires_in":3600,
  "refresh_token":"REFRESH_TOKEN",
  "scope":"account.read offline",
  "token_type":"bearer"
}
```
You can now use the `access_token` to make API calls to Frame.io on the logged-in user's behalf, and the `refresh_token` to request a new `access_token` after this token expires.