[Python-3000] String formating operations in python 3k (original) (raw)

Ian Bicking ianb at colorstudy.com
Mon Apr 3 18:13:28 CEST 2006

Barry Warsaw wrote:

On Mon, 2006-04-03 at 23:43 +1000, Nick Coghlan wrote:

What do you think of a "format" builtin function that accepts the format as the first argument (similar to printf). The version on my harddrive permits positional arguments via 1,1, 1,2, etc, as well as string formatting (by sticking the format code in square brackets between the $ and the identifier). Keyword arguments still work, naturally. And if you don't want formatting, you just leave out the square brackets. I'm not totally sure I would need a builtin. If I look at the two sources of $-strings in an app like Mailman, I'll see 1) literal human readable/translatable strings in the source code, 2) human entered strings that come from a web form. In the first case, all string formatting will funnel through one bottleneck function, which will do the catalog lookup, frame variable discovery, and substitution all in one fell swoop. So there, the builtin doesn't buy you much convenience.

Well, error messages are a common place I use %. So:

assert path.startswith(prefix), ( "%r should start with %r" % (path, prefix)) assert path.startswith(prefix), ( """{repr(path)} should start with ${repr(prefix)}") assert path.startswith(prefix), ( "$path should start with $prefix".substitute( path=repr(path), prefix=repr(prefix))

The second example assumes that you can include full expressions, otherwise that example would start looking really unpleasant. As it is, the first example still looks more-or-less the best, and it's too bad $-based substitution doesn't include it. I'm not sure how it would include it, unless there was something like =path,or=path, or =path,or, or $path or something. I dunno, none of those are very appealing.

str.substitute isn't too bad, except that "substitute" feels a little long-winded (compared to, say, str.format). If "$path".format() worked, that'd be nice, but even though it's not that hard to implement, it breaks a lot of expectations.

Anyway, that's one example. Any non-i18n'd code will likely have a fair amount of string substitution which isn't otherwise wrapped in any routines.

-- Ian Bicking / ianb at colorstudy.com / http://blog.ianbicking.org

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