Wordpress - How to include a file using get_template_part() in a plugin?

get_template_part is a theme function. You can't load plugin files with that function. Take a look at the source and you will notice the work is done by locate_template. Look at that source and you will see that it always loads from theme directories.

However much you may want to use get_template_part it is the wrong function.

You will need to include your files.

The reason, so it seems to me, for get_template_part is to allow themes to be extended-- aka, to ease the creation of child themes. Plugins are not intended to be extended in that way so there is no need for get_template_part or for any plugin equivalent.

@s_ha_dum is correct that get_template_part is a theme function, but he is incorrect that plugins are not intended to be extended in this way. It is simply more complicated.

This post by Pippin, describes how to use a function that will do the work of loading your plugin templates, while allowing users to override your plugin templates within their theme .

Essentially, it looks in a special folder in the theme, then if not found there, it looks within the templates folder for the plugin.

As was said before, you can't use get_template_part in plugins, but there's a handy class on Github (created by Gary Jones) that mimics the get_template_part functionality in plugins, adding the plugin to the fallback (child theme > parent theme > plugin).

In this way, you can override your plugin’s ”template part“ inside a child theme or a parent theme.

Usage (taken from the Github repo instructions):

1. Copy class-gamajo-template-loader.php into your plugin. It can be into a file in the plugin root, or better, an includes directory.
2. Create a new file, such as class-your-plugin-template-loader.php, in the same directory.
3. Create a class in that file that extends Gamajo_Template_Loader.
4. Override the class properties to suit your plugin. You could also override the get_templates_dir() method if it isn't right for you.
5. You can now instantiate your custom template loader class, and use it to call the get_template_part() method. This could be within a shortcode callback, or something you want theme developers to include in their files.

Example code:

// Template loader instantiated elsewhere, such as the main plugin file.
$meal_planner_template_loader = new Meal_Planner_Template_Loader;

// Use it to call the get_template_part() method. This could be within 
// a shortcode callback, or something you want theme developers 
// to include in their files.
$meal_planner_template_loader->get_template_part( 'recipe' );

// If you want to pass data to the template, call the set_template_data() 
// method with an array before calling get_template_part().        
// set_template_data() returns the loader object to allow for method chaining.
$data = array( 'foo' => 'bar', 'baz' => 'boom' );

$meal_planner_template_loader
    ->set_template_data( $data );
    ->get_template_part( 'recipe' );

// The value of bar is now available inside the recipe template as $data->foo.
// If you wish to use a different variable name, add a second parameter 
// to set_template_data():
$data = array( 'foo' => 'bar', 'baz' => 'boom' );

$meal_planner_template_loader
    ->set_template_data( $data, 'context' )
    ->get_template_part( 'recipe', 'ingredients' );

// The value of bar is now available inside the recipe template as $context->foo.