import uuid


def request_headers(headers=None):
    """Return dictionary of default HTTP headers for Graph API calls.
    Optional argument is other headers to merge/override defaults."""
    default_headers = {'SdkVersion': 'sample-python-flask',
                       'x-client-SKU': 'sample-python-flask',
                       'client-request-id': str(uuid.uuid4()),
                       'return-client-request-id': 'true'}
    if headers:
        default_headers.update(headers)
    return default_headers