.php

<?php
class TemplatePart {
	protected $template;
	protected $slug;
	protected $vars = array();
	protected $WP_Query;

	public function __construct( $template, $slug = '' ) {
		global $wp_query;

		$this->template = $template;
		$this->slug     = $slug;
		$this->WP_Query = $wp_query;
	}

	public function set_var( $key, $value ) {
		if ( null === $this->WP_Query->get( $key, null ) || ! isset( $this->vars[$key] ) ) {
			$this->vars[$key] = $value;
			$this->WP_Query->set( $key, $value );
		}
	}

	public function set_vars( $vars ) {
		foreach ( $vars as $key => $value ) {
			$this->set_var( $key, $value );
		}
	}

	public function render() {
		get_template_part( $this->template, $this->slug );
		foreach ( $this->vars as $key => $value ) {
			$this->WP_Query->set( $key, null );
		}
	}
}


$vars = [
  '_foo' => 'foo',
  '_bar' => 'bar',
];
$TemplatePart = new TemplatePart( 'template-parts/foo' );
$TemplatePart->set_vars( $vars );
$TemplatePart->render();