Prepare the access token request.

    The client makes a request to the token endpoint by adding the
    following parameters using the ``application/x-www-form-urlencoded``
    format in the HTTP request entity-body:

    :param grant_type: To indicate grant type being used, i.e. "password",
                       "authorization_code" or "client_credentials".

    :param body: Existing request body (URL encoded string) to embed parameters
                 into. This may contain extra parameters. Default ''.

    :param include_client_id: `True` (default) to send the `client_id` in the
                              body of the upstream request. This is required
                              if the client is not authenticating with the
                              authorization server as described in
                              `Section 3.2.1`_.
    :type include_client_id: Boolean

    :param client_id: Unicode client identifier. Will only appear if
                      `include_client_id` is True. *

    :param client_secret: Unicode client secret. Will only appear if set to a
                          value that is not `None`. Invoking this function with
                          an empty string will send an empty `client_secret`
                          value to the server. *

    :param code: If using authorization_code grant, pass the previously
                 obtained authorization code as the ``code`` argument. *

    :param redirect_uri: If the "redirect_uri" parameter was included in the
                         authorization request as described in
                         `Section 4.1.1`_, and their values MUST be identical. *

    :param kwargs: Extra arguments to embed in the request body.

    Parameters marked with a `*` above are not explicit arguments in the
    function signature, but are specially documented arguments for items
    appearing in the generic `**kwargs` keyworded input.

    An example of an authorization code token request body:

    .. code-block:: http

        grant_type=authorization_code&code=SplxlOBeZQQYbYS6WxSbIA
        &redirect_uri=https%3A%2F%2Fclient%2Eexample%2Ecom%2Fcb

    .. _`Section 4.1.1`: https://tools.ietf.org/html/rfc6749#section-4.1.1

def prepare_token_request(grant_type, body='', include_client_id=True, **kwargs):
    """Prepare the access token request.

    The client makes a request to the token endpoint by adding the
    following parameters using the ``application/x-www-form-urlencoded``
    format in the HTTP request entity-body:

    :param grant_type: To indicate grant type being used, i.e. "password",
                       "authorization_code" or "client_credentials".

    :param body: Existing request body (URL encoded string) to embed parameters
                 into. This may contain extra parameters. Default ''.

    :param include_client_id: `True` (default) to send the `client_id` in the
                              body of the upstream request. This is required
                              if the client is not authenticating with the
                              authorization server as described in
                              `Section 3.2.1`_.
    :type include_client_id: Boolean

    :param client_id: Unicode client identifier. Will only appear if
                      `include_client_id` is True. *

    :param client_secret: Unicode client secret. Will only appear if set to a
                          value that is not `None`. Invoking this function with
                          an empty string will send an empty `client_secret`
                          value to the server. *

    :param code: If using authorization_code grant, pass the previously
                 obtained authorization code as the ``code`` argument. *

    :param redirect_uri: If the "redirect_uri" parameter was included in the
                         authorization request as described in
                         `Section 4.1.1`_, and their values MUST be identical. *

    :param kwargs: Extra arguments to embed in the request body.

    Parameters marked with a `*` above are not explicit arguments in the
    function signature, but are specially documented arguments for items
    appearing in the generic `**kwargs` keyworded input.

    An example of an authorization code token request body:

    .. code-block:: http

        grant_type=authorization_code&code=SplxlOBeZQQYbYS6WxSbIA
        &redirect_uri=https%3A%2F%2Fclient%2Eexample%2Ecom%2Fcb

    .. _`Section 4.1.1`: https://tools.ietf.org/html/rfc6749#section-4.1.1
    """
    params = [('grant_type', grant_type)]

    if 'scope' in kwargs:
        kwargs['scope'] = list_to_scope(kwargs['scope'])

    # pull the `client_id` out of the kwargs.
    client_id = kwargs.pop('client_id', None)
    if include_client_id:
        if client_id is not None:
            params.append((unicode_type('client_id'), client_id))

    # the kwargs iteration below only supports including boolean truth (truthy)
    # values, but some servers may require an empty string for `client_secret`
    client_secret = kwargs.pop('client_secret', None)
    if client_secret is not None:
        params.append((unicode_type('client_secret'), client_secret))

    # this handles: `code`, `redirect_uri`, and other undocumented params
    for k in kwargs:
        if kwargs[k]:
            params.append((unicode_type(k), kwargs[k]))

    return add_params_to_qs(body, params)