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Date: Fri, 23 Dec 2011 17:02:18 +1100
From: Cameron Simpson <cs@zip.com.au>
To: Tim Chase <python.list@tim.thechases.com>
Subject: Re: Idiom for shelling out to $EDITOR/$PAGER?
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On 22Dec2011 22:16, Tim Chase <python.list@tim.thechases.com> wrote:
| After a little searching, I've not been able to come up with what
| I'd consider canonical examples of consider calling an external
| editor/pager on a file and reading the results back in.  (most of my
| results are swamped by people asking about editors written in
| Python, or what the best editors for Python code are)
| 
| The pseudocode would be something like
| 
|   def edit_text(data):
|     temp_fname = generate_temp_name()
|     try:
|       f = file(temp_fname, 'w')
|       f.write(data)
|       f.close()
|       before = info(temp_fname) # maybe stat+checksum?
|       editor = find_sensible_editor()
|       subprocess.call([editor, temp_fname])
|       if before == info(temp_fname):
|         return None
|       else:
|         return file(temp_fname).read()
|     finally:
|       delete_if_exists(temp_fname)
| 
| However there are things to watch out for in this lousy code:
| 
| -race conditions, unique naming, and permissions on the temp file

NamedTemporaryFile is your friend.

| -proper & efficient detection of file-change, to know whether the
| user actually did anything

Wait for the editor to exit?
In that scenario I go for:
  - wait for edit to exit
  - if exit status 0 and file non-empty, trust it
    (subject to parse issues afterwards of course)

| -cross-platform determination of a sensible editor (that blocks
| rather than spawns), using platform conventions like
| os.environ['EDITOR']

os.environment.get('EDITOR', 'vi')?

Some platforms have an executable called "editor" that solves that
problem (absent $EDITOR); MacOSX has "open", though it won't be running
a blocking editor, alas.

You may need some knowledge of the local system. On a terminal? Easy,
use $EDITOR. No terminal, but $DISPLAY? xterm -e "$EDITOR temp_file",
or the like. No terminal, no $DISPLAY, macosx? I have an incantation
somewhere...

| -cleanup deletion of the temp-file

NamedTemporaryFile is your friend.

| I presume the code for spawning $PAGER on some content would look
| pretty similar.

Yep.

| Any good example code (or blog posts, or other links) that has been
| battle-tested?

Hmm. Nothing directly to had, but regarding the temp file:

  # write contents of the file `fp` into a temp file
  with NamedTemporaryFile('w', dir=os.path.join(self.dir, 'tmp')) as T:
    T.write(fp.read())
  ... do stuff with T.name (the temp file name).

Anyway, look it up; it has an autodelete mode etc.

Cheers,
-- 
Cameron Simpson <cs@zip.com.au> DoD#743
http://www.cskk.ezoshosting.com/cs/

I have always been a welly man myself. They are superb in wet grass, let
alone lagoons of pig shit.      - Julian Macassey