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Sun Nov 30 20:08:14 EST 2003 · https://mail.python.org/pipermail/python-dev/2003-November/040541.html

> I don't know if it should be *that* mechanical; there are a lot of
> places where I've seen " 'something %s' % repr(foo)" when I think
> it's much more clearly written as " 'something %r' % foo".  I don't
> know which is the officially preferred style, but if it's the latter
> (and I hope it is ;)) then it may not be good to mechanically change
> backticks to a repr call.

If you're going to do that, I would beware of one thing.  If x is a
tuple, "foo %r" % x will not do the right thing: it will expect x to
be a 1-tuple and produce the repr of x[0]:

>>> a = (42,)
>>> print "foo %s" % repr(a)
foo (42,)
>>> print "foo %r" % a
foo 42
>>> a = (4, 2)
>>> print "foo %r" % a
Traceback (most recent call last):
  File "<stdin>", line 1, in ?
TypeError: not all arguments converted during string formatting
>>> 

This is only a problem when there's only one % format in the string;
if there are two or more, the argument is already a tuple and the
substitution of %s/repr(x) to %r/x works fine.  This also suggests a
solution: if there's only one argument, create an explicit tuple:

>>> print "foo %r" % (a,)
foo (4, 2)
>>> 

--Guido van Rossum (home page: http://www.python.org/~guido/)

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[Python-Dev] Banishing apply(), buffer(), coerce(), and intern()