How to automatically install Emacs packages by specifying a list of package names?

Question

I am using package to manage my Emacs extensions. In order to synchronize my Emacs settings on different computers, I\'d like a way to specify a list of package

Answer 1

I ran into a problem that nothing happened after adding (package-install 'org) into .emacs. I wanted to install the up-to-date version of org-mode and the built-in org-mode is quite old.

I dug out the source code of package-install from Emacs 25.3.1. The function self already checks if a package is installed or not and refuses to install it if the package is already installed. So the check (unless (package-installed-p package) ...) from answer 10093312 is in fact uncalled for.

(defun package-install (pkg &optional dont-select)
  "Install the package PKG.
PKG can be a package-desc or a symbol naming one of the available packages
in an archive in `package-archives'.  Interactively, prompt for its name.

If called interactively or if DONT-SELECT nil, add PKG to
`package-selected-packages'.

If PKG is a package-desc and it is already installed, don't try
to install it but still mark it as selected."
  (interactive
   (progn
     ;; Initialize the package system to get the list of package
     ;; symbols for completion.
     (unless package--initialized
       (package-initialize t))
     (unless package-archive-contents
       (package-refresh-contents))
     (list (intern (completing-read
                    "Install package: "
                    (delq nil
                          (mapcar (lambda (elt)
                                    (unless (package-installed-p (car elt))
                                      (symbol-name (car elt))))
                                  package-archive-contents))
                    nil t))
           nil)))
  (add-hook 'post-command-hook #'package-menu--post-refresh)
  (let ((name (if (package-desc-p pkg)
                  (package-desc-name pkg)
                pkg)))
    (unless (or dont-select (package--user-selected-p name))
      (package--save-selected-packages
       (cons name package-selected-packages)))
    (if-let ((transaction
              (if (package-desc-p pkg)
                  (unless (package-installed-p pkg)
                    (package-compute-transaction (list pkg)
                                                 (package-desc-reqs pkg)))
                (package-compute-transaction () (list (list pkg))))))
        (package-download-transaction transaction)
      (message "`%s' is already installed" name))))

The built-in org-mode also counts as installed and package-install refuses to install the newer version from ELPA. After spending some time reading package.el, I came up with the following solution.

(dolist (package (package-compute-transaction
                  () (list (list 'python '(0 25 1))
                           (list 'org '(20171211)))))
  ;; package-download-transaction may be more suitable here and
  ;; I don't have time to check it
  (package-install package))

The reason why it works is that package-* family functions handle the arguments differently based on whether if it is a symbol or a package-desc object. You can only specify version info for package-install via a package-desc object.