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[Fab-user] String interpolations

From: Niklas Lindström
Subject: [Fab-user] String interpolations
Date: Wed, 13 May 2009 15:12:35 +0200


I've finally had a bit of time to check out Fabric 0.9. It looks
great! The module reorganisation looks clean and well-thought out (and
I really like the stuff in fabric.contrib).

However, I do miss the string interpolation feature of the older
versions. I can understand if it wasn't ideal (too
implicit/unpythonic, using "non-standard" syntax etc). And I don't
mind having to do *some* explicit invokation to expand variables. But
I never was much of a fan of using string building and/or modula
formatting alá classic python *in these scenarios*. Thus I wonder if
it would be nice to reintroduce some form of utility for it?

Of course, I can just do e.g.:

    run("cd %(some_dir)s && cmd %(a_dir)s" % env)

But I *really* dislike the arcane syntax, especially the trailing "s".
And at least my fabric scripts uses interpolation quite often, which
makes it very noisy IMHO.

I tried out one version based on the str.format-stuff in Python
2.6/3.0 ([1], [2]). My take was basically:

    import sys
    def fmt(s):
        caller_locals = sys._getframe(1).f_locals
        return s.format(**caller_locals)

    class Env:
        def __init__(self):
            self.a, self.b  = "hello", "world"

    env = Env()
    c = "etc"

    print fmt("Say {env.a} {env.b}, {c}.")

This also solves a desire of mine of having a convenient way to use
both env and local vars easily in the same fashion.

Unfortunately I haven't found any pure-python implementation of the
new `str.format` method. (The one at [3] lacks dot-access.) And
dragging some other external templating into this feels too

Another option might be e.g. adding a `fmt` method on env (placing it
in env to avoid the _getframe "hack"), and use `string.Template` in
that (available since Python 2.4). It could be used like:

    run(env.fmt("cd $some_dir && cmd $a_dir"))

, optionally taking a dict arg and/or kwargs to supply more names to
interpolate.. I'd probably always do a `fmt = env.fmt` at the top of
my scripts, and use the builtin `vars()` as extra arg where needed;
but that's me. ;)

Any thoughts or alternate suggestions? I'd gladly put something
together if we reach consensus.

Best regards,

[1] = <http://www.python.org/dev/peps/pep-3101/>
[2] = <http://docs.python.org/library/string.html#new-string-formatting>
[3] = <http://svn.python.org/view/sandbox/trunk/pep3101/>

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