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Re: [Fab-user] String interpolations

From: Norman Harman
Subject: Re: [Fab-user] String interpolations
Date: Wed, 13 May 2009 11:01:52 -0500
User-agent: Thunderbird (Macintosh/20080914)

That was absolutely a neat feature. And I believe I read awhile back that the lazy evaluation has been ditched as well. That feature was so fundamental to how I wrote modular flexible fabfiles that I can't consider upgrading to a version that lacks it or equivalent functionality.

Sorry for adding nothing more than "me too" to the conversation. Fabric current rate of change is too much for me to keep up with. Maybe when it stabilizes I'll have a deeper look and see if I might be able to add some code/convince people for it's need.

Christian Vest Hansen wrote:
The syntax doesn't bother me that much. The thing I miss the most is
the recursive resolution behaviour that the old lazy_format function
exhibited. I had grown a habit of composing paths and filenames and
other strings from many parts that themselves may have been composed
of other parts. I still think that was a neat feature.

2009/5/13 Niklas Lindström <address@hidden>:
Hi Jeff,

amazingly quick response as usual! :) I also agree with your reasoning.

I just wrote a very simple but "somewhat magic" fusion of my
suggestions. I just inline here for quick evaluation::

-------------------- 8< --------------------
import sys
from string import Template
from fabric.api import env

def fmt(s, *args, **kwargs):
   This variable expansion utility attempts (using ``string.Template``) to
   substitute variable references in the supplied string. The order
of lookup is:

   1. Keyword arguments (if any).
   2. Variables in the *calling* scope.
   3. A variable defined in fabric's ``env`` namespace.


       >>> fmt("$shell $notexpanded")
       '/bin/bash -l -c $notexpanded'

       >>> shell = "local"
       >>> fmt("$shell $notexpanded")
       'local $notexpanded'

       >>> fmt("$shell $notexpanded", shell="other")
       'other $notexpanded'

       >>> fmt("$$shell $notexpanded", shell="other")
       '$shell $notexpanded'

   data = {}
   return Template(s).safe_substitute(data)
-------------------- >8 --------------------

One thing (good or bad depending on perspective) is that this makes it
possible to implicitly mix expansion and variables that should be left
for the target machine shell to expand. (You can always escape
variables with double-dollar to prevent *any* expansion on the fabric
side. Or if this behaviour should be enforced, replace
`safe_substitute` with `substitute`.)

Perhaps something like it could end up in fabric.contrib?

Best regards,

2009/5/13 Jeff Forcier <address@hidden>:
2009/5/13 Niklas Lindström <address@hidden>:

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 totally agree about how current Python string interpolation is a
little on the verbose side and could be easier to use (and that the
newer format string stuff in 2.6+ looks pretty nice); but I don't
think it's *bad enough* to merit adding a decent amount of extra
complexity to Fabric. That was basically my thinking when I decided
not to reimplement it in the 0.9 rewrite, and I still maintain it's
the way to go.

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"))
I might be OK with this approach, however: adding a convenience
function to do this special interpolation, which users may opt into
using, and which is not sitting around cluttering up the rest of the
code, would be more palatable than something with its fingers in the
majority of all string-handling functions.

Still think there needs to be a stronger argument for this existing --
i.e. for a new user reading the docs, and coming across this, what
will the docs say to justify the existence of such a function outside
of "someone didn't really like the default string interpolation

But, if this were to be implemented in some fashion I would definitely
at least consider putting it in, especially if other users would find
it useful.


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