Authentication and Client Creation

By now, you should have followed the instructions in Getting Started and are ready to start making API calls. Read this page to learn how to get over the last remaining hurdle: OAuth authentication.

Before we begin, however, note that this guide is meant to users who want to run applications on their own machines, without distributing them to others. If you plan on distributing your app, or if you plan on running it on a server and allowing access to other users, this login flow is not for you.

OAuth Refresher

This section is purely for the curious. If you already understand OAuth (wow, congrats) or if you don’t care and just want to use this package as fast as possible, feel free to skip this section. If you encounter any weird behavior, this section may help you understand that’s going on.

Webapp authentication is a complex beast. The OAuth protocol was created to allow applications to access one anothers’ APIs securely and with the minimum level of trust possible. A full treatise on this topic is well beyond the scope of this guide, but in order to alleviate some of the confusion and complexity that seems to surround this part of the API, let’s give a quick explanation of how OAuth works in the context of TD Ameritrade’s API.

The first thing to understand is that the OAuth webapp flow was created to allow client-side applications consisting of a webapp frontend and a remotely hosted backend to interact with a third party API. Unlike the backend application flow, in which the remotely hosted backend has a secret which allows it to access the API on its own behalf, the webapp flow allows either the webapp frontend or the remotely host backend to access the API on behalf of its users.

If you’ve ever installed a GitHub, Facebook, Twitter, GMail, etc. app, you’ve seen this flow. You click on the “install” link, a login window pops up, you enter your password, and you’re presented with a page that asks whether you want to grant the app access to your account.

Here’s what’s happening under the hood. The window that pops up is the authentication URL, which opens a login page for the target API. The aim is to allow the user to input their username and password without the webapp frontend or the remotely hosted backend seeing it. On web browsers, this is accomplished using the browser’s refusal to send credentials from one domain to another.

Once login here is successful, the API replies with a redirect to a URL that the remotely hosted backend controls. This is the callback URL. This redirect will contain a code which securely identifies the user to the API, embedded in the query of the request.

You might think that code is enough to access the API, and it would be if the API author were willing to sacrifice long-term security. The exact reasons why it doesn’t work involve some deep security topics like robustness against replay attacks and session duration limitation, but we’ll skip them here.

This code is useful only for fetching a token from the authentication endpoint. This token is what we want: a secure secret which the client can use to access API endpoints, and can be refreshed over time.

If you’ve gotten this far and your head isn’t spinning, you haven’t been paying attention. Security-sensitive protocols can be very complicated, and you should never build your own implementation. Fortunately there exist very robust implementations of this flow, and tda-api’s authentication module makes using them easy.

Fetching a Token and Creating a Client

tda-api provides an easy implementation of the client-side login flow in the auth package. It uses a selenium webdriver to open the TD Ameritrade authentication URL, take your login credentials, catch the post-login redirect, and fetch a reusable token. It returns a fully-configured Client Wrapper, ready to send API calls. It also handles token refreshing, and writes updated tokens to the token file.

tda.auth.client_from_login_flow(webdriver, api_key, redirect_url, token_path, redirect_wait_time_seconds=0.1)

Uses the webdriver to perform an OAuth webapp login flow and creates a client wrapped around the resulting token. The client will be configured to refresh the token as necessary, writing each updated version to token_path.

Parameters:
  • webdriver – selenium webdriver which will be used to perform the login flow.
  • api_key – Your TD Ameritrade application’s API key, also known as the client ID.
  • redirect_url – Your TD Ameritrade application’s redirect URL. Note this must exactly match the value you’ve entered in your application configuration, otherwise login will fail with a security error.
  • token_path – Path to which the new token will be written. Updated tokens will be written to this path as well.

Once you have a token written on disk, you can reuse it without going through the login flow again.

tda.auth.client_from_token_file(token_path, api_key)

Returns a session from the specified token path. The session will perform an auth refresh as needed. It will also update the token on disk whenever appropriate.

Parameters:
  • token_path – Path to the token. Updated tokens will be written to this path.
  • api_key – Your TD Ameritrade application’s API key, also known as the client ID.

The following is a convenient wrapper around these two methods, calling each when appropriate:

tda.auth.easy_client(api_key, redirect_uri, token_path, webdriver_func=None)

Convenient wrapper around client_from_login_flow() and client_from_token_file(). If token_path exists, loads the token from it. Otherwise open a login flow to fetch a new token. Returns a client configured to refresh the token to token_path.

Parameters:
  • api_key – Your TD Ameritrade application’s API key, also known as the client ID.
  • redirect_url – Your TD Ameritrade application’s redirect URL. Note this must exactly match the value you’ve entered in your application configuration, otherwise login will fail with a security error.
  • token_path – Path that new token will be read from and written to. Updated tokens will be written to this path as well.
  • webdriver_func – Function that returns a webdriver for use in fetching a new token. Will only be called if the token file cannot be found.

Troubleshooting

As simple as it seems, this process is complex and mistakes are easy to make. This section outlines some of the more common issues you might encounter. If you find yourself dealing with something that isn’t listed here, or if you try the suggested remedies and are still seeing issues, please file a ticket on our issues page.

“A third-party application may be attempting to make unauthorized access to your account”

One attack on improperly implemented OAuth login flows involves tricking a user into submitting their credentials for a real app and then redirecting to a malicious web server (remember the GET request to the redirect URI contains all credentials required to access the user’s account). This is especially pernicious because from the user’s perspective, they see a real login window and probably never realize they’ve been sent to a malicious server, especially if the landing page is designed to resemble the target API’s landing page.

TD Ameritrade correctly prevents this attack by refusing to allow a login if the redirect URI does not exactly match the redirect URI with which the app is configured. If you make any mistake in setting your API key or redirect URI, you’ll see this instead of a login page:

A third-party application may be attempting to make unauthorized access to your account

If this happens, you almost certainly copied your API key or redirect URI incorrectly. Go back to your application list and copy-paste it again.

tda-api Hangs After Successful Login

After opening the login window, tda-api loops and waits until the webdriver’s current URL starts with the given redirect URI:

callback_url = ''
while not callback_url.startswith(redirect_url):
    callback_url = webdriver.current_url
    time.sleep(redirect_wait_time_seconds)

Usually, it would be impossible for a successful post-login callback to not start with the callback URI, but there’s one major exception: when the callback URI starts with http. Behavior varies by browser and app configuration, but a callback URI starting with http can sometimes be redirected to one starting with https, in which case tda-api will never notice the redirect.

If this is happening to you, consider changing your callback URI to use https instead of http. Not only will it make your life easier here, but it is extremely bad practice to send credentials like this over an unencrypted channel like that provided by http.