Steven Barker added the comment:
The behavior of !s with the format() methods isn't exactly the same as %s with % formatting. With the latter, the conversion depends on the type of the result string, which in turn depends on whether the format string *or any of the values values* is unicode:
>>> class X():
def __str__(self): return "str"
def __unicode__(self): return u"unicode"
>>> "%s %s" % ("foo", X())
'foo str'
>>> "%s %s" % (u"foo", X())
u'foo unicode'
>>> u"%s %s" % ("foo", X())
u'foo unicode'
>>> u"%s %s" % (u"foo", X())
u'foo unicode'
The format methods are more consistent, always returning the same type as the format string regardless of the types of the arguments (and using the appropriate converter):
>>> "{} {!s}".format("foo", X())
'foo str'
>>> "{} {!s}".format(u"foo", X())
'foo str'
>>> u"{} {!s}".format("foo", X())
u'foo unicode'
>>> u"{} {!s}".format(u"foo", X())
u'foo unicode'
The documentation for %s conversion (in the second table here: https://docs.python.org/2/library/stdtypes.html#string-formatting-operations ) also suggests that it always uses str(), though the footnote for that table entry alludes to the behavior shown above without ever mentioning using unicode() for conversions explicitly.
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nosy: +Steven.Barker
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