python-mode: ensure t or nil?

Dear David,

in your Python IDE video you explained to use `ensure nil` to make sure that the currently installed (emacs in build?) pytho mode is used instead of the one on MELPA.

But in your Emacs.org file here in this repo the `ensure` is set to `t` not `nil`.

```
(use-package python-mode
  :ensure t
```

Do I misunderstood something?