[Python-3000] String formating operations in python 3k

skip at pobox.com skip at pobox.com
Mon Apr 3 22:37:21 CEST 2006

I sort of lost track of the thread, then noticed a recent message where
Guido seems to be cozying up to the idea, so I thought maybe I ought to
think about it for a few minutes.  As far as I can tell, the proposal is:

    usage               example
    s.format(x, y)      simple % substition, e.g.
                        "my %s has %s".format("dog", "fleas")
    s.format(**x)       named substitution using dictionaries, e.g.
                        pet = "kangaroo"
                        pest = "marmots"
                        "my %(pet)s has %(pest)s".format(**locals())
    s.format(key=val)   named substitution using keyword args, e.g.
                        "my %(pet)s has %(pest)s".format(pet="armadillo",

One thing that's always been a bit cumbersome for me with the current mod
operator syntax is that it's all or nothing.  I either have to cram
everything into a single dictionary, hack up some sort of multimap, or fall
back to simple substitution.  With str.format(), I could presumably do
something like

    pest = current_plague()
    if pest:
        print "my %(pet)s has %(pest)s".format(pet="dog", **locals())

That is, mix keyword-style and dict-style parameter passing.  The above is a
lame example, but I occasionally find myself creating single-use local
variables to hold arithmetic expressions I want to display.  Computing the
expression to be expanded in the function call would be cleaner.

If I have my keyword mappings in multiple dictionaries might something like

    "my %(pet)s has %(pest)s".format(**pets, **pests)

be supported?  That reminds me of the debate about adding to dictionaries:
 how are duplicate keys handled?  Multiple occurrences of **dict
aren't supported in Python 2.4.  I suppose it probably doesn't typically
make a lot of sense, but for this it seems like it might be reasonable.


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