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Re: [Fab-user] String interpolations

From: Christian Vest Hansen
Subject: Re: [Fab-user] String interpolations
Date: Wed, 13 May 2009 16:54:00 +0200

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.
>    Examples::
>        >>> 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 = {}
>    data.update(env)
>    data.update(sys._getframe(1).f_locals)
>    data.update(kwargs)
>    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,
> Niklas
> 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
>> syntax"?
>> 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.
>> Best,
>> Jeff
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Venlig hilsen / Kind regards,
Christian Vest Hansen.

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