[Python-Dev] string formatting options and removing basestring.__mod__ (WAS: Replacement for print in Python 3.0)

Steven Bethard steven.bethard at gmail.com
Mon Sep 5 00:08:18 CEST 2005

[Raymond Hettinger]
> Actually, formatting needs to become a function.  The overloading of the
> arithmetic mod operator has proven to be unfortunate (if only because of
> precedence issues).

[Guido van Rossum]
> For me, it's not so much the precedence, but the fact that "%s" % x
> doesn't work as expected if x is a tuple; you'd have to write "%s" %
> (x,) which is tedious.

[Raymond Hettinger]
> Also, the format coding scheme itself needs to be revisited.  There is
> no shortage of people who have taken issue with the trailing s in
> %(myvar)s.
> string.Template is a bit too simplified.  But perhaps it can be adapted.
> We still want some way to express %r, %6.2f, etc.    Since string
> formatting has been around since Tim was in diapers, we should probably
> start by looking at the solutions used by other languages.

I was curious about what kind of options there were, so I googled
around a bit.  Here's what I found:

Java[1] uses syntax like:
which is basically the same as that of C, with positional argument
specifiers.  Some examples:
    String.format("Duke's Birthday: %1$tm %1$te,%1$tY", c);
    System.out.format("Local time: %tT", Calendar.getInstance());
    formatter.format("%4$2s %3$2s %2$2s %1$2s", "a", "b", "c", "d")
Classes can customize formatting for the 's' specifier by implementing
the Formattable interface, which provides a method:
    formatTo(Formatter fmt, int f, int width, int precision)
You can get formatted objects by calling:
* The format() methods on Formatter objects
* The format() methods on Strings
* The format() methods on System.out, System.err, etc.

.Net[2] uses syntax like:
with examples like:
    String.Format("{0:dddd MMMM}", DateTime.Now)
    Console.WriteLine("{0:C}", MyInt)
    String.Format("Name = {0}, hours = {1:hh}, minutes = {1:mm}",
          myName, DateTime.Now)
Classes can customize formatting for any specifier by implementing the
ICustomFormatter interface:
    Format(string format, object arg, IFormatProvider formatProvider);
or the IFormattable interface:
    ToString(string format, IFormatProvider formatProvider);
You can get formatted objects by calling:
* The ToString method of an IFormattable instance
* The Format on Strings
* The Write and WriteLine methods of Console, TextWriter,
StreamWriter, etc. objects

I also briefly looked at Tcl/Tk[3], Common Dylan[4], OCaml[5] and
Ruby[6][7], which all appear to use C-style (or similar) formatting. 
I believe that Ruby, in addition to having printf and sprintf, also
uses the % operator like Python does.

This was a pretty small sample of languages (just the first few that
showed up in google), and I didn't really look at any of them other
than Java and .Net in much depth, so I've may have misunderstood some
of it.  That said, I think it's probably pretty reasonable to conclude
that C-style formatting is the choice of a lot of other languages. 
(Not to imply that it therefore needs to be the choice of Python.)

I understand one of the complaints about string formatting in Python
is having to write the "s" on things like "%(key)s".  I was hoping to
get some ideas for alternatives here, but I wasn't able to find any
dict-style insertion like in Python.  There were a few languages
(Java, Tcl) with the N$ positional-style insertion, but I don't think
that helps us much.

People have also been discussing a builtin format() function to
replace the current % operator.  Translating into Python the location
of the formatting operations in the languages above suggests the
following possibilities:

* Have all __str__() methods take additonal formatting arguments
* Add a format() builtin
* Add format() methods on str and unicode objects
* Add format() methods on all Python streams (files, sys.stdin, etc.)

Of course, these possibilities aren't mutually exclusive, but TOOWTDI
suggests that we probably shouldn't have too many of them.

If people know of other languages that have a different approach to
string formatting, it might be useful to see them.  BTW, I
intentionally didn't go into Perl's string formatting because I don't
know it that well, and figured there are people on this list much more
qualified than myself to present it.


You can wordify anything if you just verb it.
        --- Bucky Katt, Get Fuzzy

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