Why can't Django find secrets.json file?

Question

So I have my Django project, and am trying to deploy it via heroku server. I realized that I need to created secrets.json file (in the same directory as manage.py), and then add it to .gitignore, for sake of security. But after deploying the project, I got an error saying it couldn't find secrets.json file. Below is my code.
settings.py

secret_file = os.path.join(BASE_DIR, 'secrets.json')
with open(secret_file) as f:
    secrets = json.loads(f.read())

def get_secret(setting, secrets=secrets):
    try:
        return secrets[setting]
    except KeyError:
        error_msg = "Set the {} environment variable".format(setting)
        raise ImproperlyConfigured(error_msg)
SECRET_KEY = get_secret("SECRET_KEY")

secrets.json

{
    "SECRET_KEY" : xxx,
    "EMAIL_HOST_USER" : xxx,
    "EMAIL_HOST_PASSWORD" : xxx
}

After saving these, I added secrets.json to .gitignore.
But an error occurs at line 2 (with open(secret_file) as f:), saying it can't find secrets.json file. What do you think is the problem?

*FYI, The BASE_DIR doesn't seem to be the problem, since it works fine else where. Such as STATICFILES_DIRS.