Sometimes it's annoying having to manually select a variable name and copy it, especially when you deal with multiple languages.

Apart from manually selecting the text you want, you try to use mark-sexp, which works fine but fails in the following python example when the cursor is at the character "p". 

report_prefix = "report name"

In the above example, only prefix will be selected with mark-sexp.

There are many ways to solve this problem (e.g. use ivy-thing-at-point). However I found using tree-sitter and emacs-tree-sitter to be the fastest and most accurate solution.

Here's the function I wrote that implements this. 

(require 'tree-sitter)
(defun mark-node ()
  "Mark the current node under cursor using tree-sitter."
  (interactive)
  (let* ((node (tree-sitter-node-at-pos))
         (start-pos (tsc-node-start-position node))
        (end-pos (tsc-node-end-position node)))
    (goto-char start-pos)
    (push-mark end-pos)
    (setq mark-active t)))

Now if you run mark-node when your cursor is under the character "p", it'll still select report_prefix as expected. What's better is you can even use this to select "report name" wherever your cursor is at, as long as it's within the quotes. All because tree-sitter recognises that it is a string in python. 

report_prefix = "report name"