How can I get post id from my html form to views.py

I am using Django, jQuery and Ajax. But, I am confused about how I can get the post id in ajax data to use that in views.py. I am adding code here and must read comments in the code so you can understand better what i am actually trying to explain or what problem I am facing. If this question require your little bit more time than answering other questions than please do not skip this question if you know the solution. All i can do for you is that i can up vote you 10 to 15 answers so that your reputation can increase.

I am beginner with JQuery so please explain your answer briefly.

So ,Here down below i have div tag which will provide me post id. if user click on the reply button.

<div id='post_id' post="{{post.id}}">    
            {% if node.level < 3 %} 
            <button class='btn btn-success' onclick="myFunction({{node.id}})">Reply</button>
            {% endif %}
            </div>  

Than I have also form for comment.

<form id='commentform' class='commentform' method='POST'>
{% csrf_token %}
{% with allcomments as total_comments %}
<p>{{ total_comments }} comment{{total_comments|pluralize}}</p>
{% endwith %}

<select name='post' class='d-none' id='id_post'>
  <option value="{{ post.id }}" selected="{{ post.id }}"></option>
</select>

<label class='small font-weight-bold'>{{comment_form.parent.label}}</label>
{{comment_form.parent}}
<div class='d-flex'>
<img class='avatar_comment align-self-center' src="{% for data in avatar %}{{data.avatar.url}}{%endfor%}">
{{comment_form.body }}
</div>
<div class='d-flex flex-row-reverse'>
<button type='submit' class='newcomment btn btn-primary' value='commentform' id='newcomment'>Submit</button>
</div>
</form>

between script tags I have set an event which is linked with form.

 $(document).on('click', '#newcomment, #newcommentinner', function (e) {
    e.preventDefault();

    var button = $(this).attr("value");
    var post_id = document.getElementById('post_id').getAttribute('post'); #Here I am trying to take post id from div tag with id='post_id'.
    console.log(post_id,'postid') #In console it is returning me 2 which is right post id.
    var placement = "commentform"
    if (button == "newcommentform") {
      var placement = "newcommentform"
    }
 
    $.ajax({
      type: 'POST',
      url: '{% url "posts:addcomment" pk=post.pk slug=post.slug %}',
      data: $("#" + button).serialize() + {'post_id' : post_id},#Here I am trying to take that post id in data so i can use that in views.py. But in views.py it is returning me none And I don't understand why ? Because this is a post_id variable which is returning me 2 in console but in terminal it is returning me none. Please tell me how can i fix it. 
      cache: false,
      success: function (json) {
        console.log(json)

        $('<div id="" class="my-2 p-2" style="border: 1px solid grey"> \
          <div class="d-flex justify-content-between">By ' + json['user'] + '<div></div>Posted: Just now!</div> \
          <div>' + json['result2'] + '</div> \
          <hr> \
          </div>').insertBefore('#' + placement);

        $('.commentform').trigger("reset");
        formExit()
      },
      error: function (xhr, errmsg, err) {
      }
    });
  })

If more information is require than tell me. I will update my question with that information.

in console:

in terminal: