[Fab-user] RFC: Implementing a current working directory / path refactoring feature

[Jeff Forcier]

Hi all,

Ran across this fabfile in IRC today: http://gist.github.com/27401.

Not faulting the file itself (author knows it's not great) but I think
it would be nice to help users refactor their paths, and/or prevent
the sorts of hackery like on line 9 above. Stuff like 'run("cd
/foo/bar; do_stuff")' for commands that, for better or worse, need to
run from a certain location on the filesystem. Or, just generally
preventing a lot of repetitions of the same base path, as in lines
18-25.

These two problems overlap a lot, though not completely (i.e. in some
situations you DO need the full path, for e.g. symlinking a file
that's in a totally different directory tree) but I think that we can
still handle both with the same implementation:

- Set up a new variable, 'cwd' or 'fab_cwd' (former is a bit shorter
which is good when the user needs to access its value directly; latter
is more in line with other fab-specific variables).
- Allow this variable to be set somehow (see below).
- Update run() and sudo() (and possibly local()?) to implicitly
prepend it to their commands, when it's non-empty.
    - Probably with '&&', which is much safer than ';' -- imagine when
the user attempts to do the equivalent of "cd /var/www/mysite/subdir/
; rm -rf *", and then imagine that that directory does not exist.
Whoops, bye-bye home directory contents...

With such a setup, users can set the variable for certain blocks of
code (again see below) which obviates problem #1 (users having to
repeat calls to 'cd foo ; do something') and problem #2 (many of the
stuff in lines 18-25 could be simplified by factoring out
/var/www/eflorenzano.com and just running e.g. 'ln subdir/foo foo';
and elsewhere, the user can just grab the value of 'fab_cwd' to build
parts of the command).


The main question is then how to control scoping of this. Ruby,
incidentally, has a leg up here due to how it allows lots of nested
anonymous code blocks. In Python, I think we have to either use the
brand-new-ish 'with' statement (2.5+ and 2.5 requires a __future__
import) or fall back to semi ugly "flat" statefulness (i.e. calling
'cd("/var/www")', running a few lines all "in" that dir, then 'cd("")'
to 'undo' it).

So, Python 2.5+:

    run('some command in my home directory')

    with cd('/var/www/'):
        run('some other command that is now in /var/www')
        run('yet another command in /var/www')

    run('final command, which is back in home directory')

Python 2.4-:

    run('some command in my home directory')

    cd('/var/www/')
    run('some other command that is now in /var/www')
    run('yet another command in /var/www')

    cd('')
    run('final command, which is back in home directory')


Any comments/questions on all this? I'm not 100% committed to it --
it's not THAT hard for users to just set a variable to the path prefix
and do the cd;command trick where necessary -- but I think it'd be a
nice tool to have.

-Jeff