Source

File: wp-includes/post-formats.php

function set_post_format( $post, $format ) {
	$post = get_post( $post );

	if ( ! $post ) {
		return new WP_Error( 'invalid_post', __( 'Invalid post.' ) );
	}

	if ( ! empty( $format ) ) {
		$format = sanitize_key( $format );
		if ( 'standard' === $format || ! in_array( $format, get_post_format_slugs(), true ) ) {
			$format = '';
		} else {
			$format = 'post-format-' . $format;
		}
	}

	return wp_set_post_terms( $post->ID, $format, 'post_format' );
}