wp img alt always filled in

<?php

function myprefix_get_attachment_alt( $attachment_ID ) {

	// Get ALT
	$thumb_alt = get_post_meta( $attachment_ID, '_wp_attachment_image_alt', true );
	
	// No ALT supplied get attachment info
	if ( empty( $thumb_alt ) )
		$attachment = get_post( $attachment_ID );
	
	// Use caption if no ALT supplied
	if ( empty( $thumb_alt ) )
		$thumb_alt = $attachment->post_excerpt;
	
	// Use title if no caption supplied either
	if ( empty( $thumb_alt ) )
		$thumb_alt = $attachment->post_title;

	// Return ALT
	return esc_attr( trim( strip_tags( $thumb_alt ) ) );
}

// possible way of using

?>

<img src="..." alt="<?php echo myprefix_get_attachment_alt( 10 ); ?>">