[New-bugs-announce] [issue34425] :s formatting broken for objects without __format__

[Jason Spencer]

New submission from Jason Spencer <jasonspencer at google.com>:

Objects with __str__ but WITHOUT __format__ are string-convertible and default-to-str formattable.  But the explicit use of '{:s}' as a format string fails to format these objects as expected.  Either it is no longer the case that '{}' and '{:s}' are equivalent format strings, or formatting for {:s} is broken.  Users would not expect the following to be true.

(Tested in 3.5.3 and 3.6.  Maybe be the same in later releases.)

Python 3.5.3 (default, Jan 19 2017, 14:11:04) 
[GCC 6.3.0 20170118] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> class Coord:
...   def __init__(self, a, b):
...     self.a=a
...     self.b=b
...   def __str__(self):
...     return '{:d}:{:d}'.format(self.a, self.b)
... 
>>> c = Coord(3,4)
>>> str(c)
'3:4'
>>> '{:s}'.format(c)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: unsupported format string passed to Coord.__format__
>>> '{}'.format(c)
'3:4'
>>> '{!s:s}'.format(c)
'3:4'
>>>

----------
components: Interpreter Core
messages: 323688
nosy: Jason Spencer
priority: normal
severity: normal
status: open
title: :s formatting broken for objects without __format__
type: behavior
versions: Python 3.5, Python 3.6

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Python tracker <report at bugs.python.org>
<https://bugs.python.org/issue34425>
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