[issue21547] '!s' formatting documentation bug
New submission from Joshua Landau:
In the docs for 2.x about the formatting syntax:
https://docs.python.org/2/library/string.html#format-string-syntax
it says
"Two conversion flags are currently supported: '!s' which calls str() on the value, and '!r' which calls repr()."
but for unicode formatters, '!s' calls unicode() instead.
See http://stackoverflow.com/questions/23773816/why-python-str-format-doesnt-cal... for the question that found this.
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assignee: docs@python
components: Documentation
messages: 218863
nosy: Joshua.Landau, docs@python
priority: normal
severity: normal
status: open
title: '!s' formatting documentation bug
versions: Python 2.7
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Python tracker
Eric V. Smith added the comment: I suggest using whatever language explains what "u'%s' %obj" does. It's the same behavior.
From the SO question, given:
class A(object): def __str__(self): return 'as str' def __unicode__(self): return u'as unicode' Then:
'%s' % A() 'as str' u'%s' % A() u'as unicode'
and:
'{!s}'.format(A()) 'as str' u'{!s}'.format(A()) u'as unicode'
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nosy: +eric.smith
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Python tracker
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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Python tracker
Changes by Ezio Melotti
Serhiy Storchaka
participants (5)
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Eric V. Smith
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Ezio Melotti
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Joshua Landau
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Serhiy Storchaka
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Steven Barker