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Wed Jan 15 17:33:18 CET 2014 · https://mail.python.org/pipermail/python-dev/2014-January/131636.html

On 14/01/2014 19:56, Ethan Furman wrote:
> Duh.  Here's the text, as well.  ;)
>
> %s, because it is the most general, has the most convoluted resolution:
>
>    - input type is bytes?
>      pass it straight through
>
>    - input type is numeric?
>      use its __xxx__ [1] [2] method and ascii-encode it (strictly)
>
>    - input type is something else?
>      use its __bytes__ method; if there isn't one, raise an exception [3]
>
> Examples:
>
>      >>> b'%s' % b'abc'
>      b'abc'
>
>      >>> b'%s' % 3.14
>      b'3.14'
>
>      >>> b'%s' % 'hello world!'
>      Traceback (most recent call last):
>      ...
>      TypeError: 'hello world' has no __bytes__ method, perhaps you need
> to encode it?
>

For completeness I believe %r and %a should be included here as well. 
FTR %a appears to have been introduced in 3.2, but I couldn't find 
anything in the What's New and there's no note in the docs 
http://docs.python.org/3/library/stdtypes.html#printf-style-string-formatting 
to indicate when it first came into play.

-- 
My fellow Pythonistas, ask not what our language can do for you, ask 
what you can do for our language.

Mark Lawrence

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Showing content from https://mail.python.org/pipermail/python-dev/2014-January/131636.html below:

[Python-Dev] PEP 461 - Adding % and {} formatting to bytes