def _image_tag_exists(client, repo_name, image_tag):
    """
    Checks if a image tag exists
    :param client: boto3.client The ECR client 
    :param repo_name: str The name of the repository to check
    :param image_tag: str The image tag to check
    """
    images = client.list_images(repositoryName=repo_name)
    # return empty dictionary if key not found
    for i in images.get('imageIds', {}):
        # if empty dict is returned, loop skipped and False returned
        if i.get('imageTag') == image_tag:
            return True
    return False