Passing data from Partial View to its parent View

You could share state between views using the HttpContext.

@{
    this.ViewContext.HttpContext.Items["Stuff"] = "some-data";
}

and then:

@{ var result = Html.Partial("_PartialView"); }

<div id="@this.ViewContext.HttpContext.Items["Stuff"]">
    @result
<div>

Except that the example you have shown in your question:

<div id="@someDataFromPartialSomehow">
    @Html.Partial("_PartialView")
</div>

you are attempting to use the someDataFromPartialSomehow even BEFORE invoking the partial view which obviously is impossible.

Also bear in mind that what you are trying to achieve is bad design. If a partial view can only work in the context of some specific parent, then you might need to rethink your separation of views. Partial views is something that must be INDEPENDENT and REUSABLE, no matter in which context it is being placed. If it assumes things about the hosting parent then there's a serious design problem here.

I have a suggestion for you.

Put hidden input fields in the partial view and get them from javascript.

Ex: In _PartialView.cshtml

<input type="hidden" id="someDataFromPartialSomehow" value="5" />

In your view

<script>
$(document).ready(function(){
   var someDataFromPartialSomehow = $("#someDataFromPartialSomehow").val();
});
</script>

Note that you have to write the js function inside the document ready function because the partial view should be fully loaded.