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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def read_worker_creds(key="credentials"):
 """Get credentials from CREDS_FILES or the environment. This looks at the CREDS_FILES in order, and falls back to the environment. Args: key (str, optional):
 each CREDS_FILE is a json dict. This key's value contains the credentials. Defaults to 'credentials'. Returns: dict: the credentials found. None if no credentials found. """

    for path in CREDS_FILES:
        if not os.path.exists(path):
            continue
        contents = load_json_or_yaml(path, is_path=True, exception=None)
        if contents.get(key):
            return contents[key]
    else:
        if key == "credentials" and os.environ.get("TASKCLUSTER_ACCESS_TOKEN") and \
                os.environ.get("TASKCLUSTER_CLIENT_ID"):
            credentials = {
                "accessToken": os.environ["TASKCLUSTER_ACCESS_TOKEN"],
                "clientId": os.environ["TASKCLUSTER_CLIENT_ID"],
            }
            if os.environ.get("TASKCLUSTER_CERTIFICATE"):
                credentials['certificate'] = os.environ['TASKCLUSTER_CERTIFICATE']
            return credentials