[issue21547] '!s' formatting documentation bug

[Serhiy Storchaka]

Serhiy Storchaka  added the comment:

Python 2.7 is no longer supported.

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nosy: +serhiy.storchaka
resolution:  -> out of date
stage:  -> resolved
status: open -> closed

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[issue21547] '!s' formatting documentation bug

[Ezio Melotti]

Changes by Ezio Melotti ezio.melo...@gmail.com:


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type:  - behavior

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[issue21547] '!s' formatting documentation bug

[Joshua Landau]

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-call-str
 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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[issue21547] '!s' formatting documentation bug

[Eric V. Smith]

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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[issue21547] '!s' formatting documentation bug

[Steven Barker]

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 uunicode

 %s %s % (foo, X())
'foo str'
 %s %s % (ufoo, X())
u'foo unicode'
 u%s %s % (foo, X())
u'foo unicode'
 u%s %s % (ufoo, 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(ufoo, X())
'foo str'
 u{} {!s}.format(foo, X())
u'foo unicode'
 u{} {!s}.format(ufoo, 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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