Is there command to cause emacs to rescan the buffer to determine mode?

Question

Apologies if this has been asked before (I couldn't find an answer to this.)

Frequently, when I open an existing file or create a new file with emacs, the mode is determined correctly based on the file extension (text following the last "." in the file name). However, this doesn't work when I create a script that has a "shebang" ("#!") on the first line. For example,

#!/usr/bin/env python

Emacs doesn't put the buffer into python mode unless I save the file (save-file) and open it again (find-file). Is there a way to rescan the buffer to determine the mode without saving the file?

Another scenario where this would come in handy is when the buffer has:

; -*- <MODE> -*-

on the first line.

Answer 1

I believe the command you want is normal-mode.

Answer 2

normal-mode (cited by @Omar in the alternative answer) calls, in part, set-auto-mode. The non-interactive function set-auto-mode, which can be evaluated by typing M-: and then (set-auto-mode), or by typing M-x eval-expression RET (set-auto-mode) RET, places the buffer in the proper major-mode based upon the shebang line provided that said line matches the expected format ... (infra). normal-mode; however, is interactive and contains some additional goodies such as killing all local variables, running certain hooks, etc.

The variable auto-mode-interpreter-regexp, defined in files.el has a default value of:

 "#![ \t]?\\([^ \t\n]*/bin/env[ \t]\\)?\\([^ \t\n]+\\)"

We note that there is no space between the # and the ! symbol, whereas the O.P. is using a space between the aforementioned symbols; i.e., # !/usr/bin/env python. Emacs is unable to match the shebang line of the O.P. against the default regexp because of the aforementioned unexpected space between the # and ! symbols at the outset of said shebang line.

The doc-string for the above-mentioned variable provides as follows:

Regexp matching interpreters, for file mode determination.
This regular expression is matched against the first line of a file
to determine the file’s mode in ‘set-auto-mode’.  If it matches, the file
is assumed to be interpreted by the interpreter matched by the second group
of the regular expression.  The mode is then determined as the mode
associated with that interpreter in `interpreter-mode-alist`.

Answer 3

M-x revert-buffer works too as it calls normal-mode internally, though normal-mode does exactly what you want.