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[Python-Dev] When val=b'', but val == b'' returns False - bytes initialization

Antoine Pitrou solipsis at pitrou.net
Wed Dec 27 12:51:15 EST 2017

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On Wed, 27 Dec 2017 17:28:41 +0100
Antoine Pitrou <solipsis at pitrou.net> wrote:
> On Wed, 27 Dec 2017 14:19:16 +0000
> Jonathan Underwood <jonathan.underwood at gmail.com> wrote:
> > Hello,
> > 
> > I am not sure if this is expected behaviour, or a bug.
> > 
> > In a C extension module, if I create and return an empty bytes object like this:
> > 
> >     val = PyBytes_FromStringAndSize (NULL, 20);
> >     Py_SIZE(val) = 0;  
> 
> I wouldn't call it "expected", but bytes objects are supposed to be
> NULL-terminated internally, so your code technically creates an invalid
> bytes object.  The NULL-terminated constraint may be relied on by some
> code, for example when the string gets passed to a third-party C
> function.

Note this is really happening because you're allocating a 20-long bytes
object and then shortening it to 0 bytes.  PyBytes_FromStringAndSize()
already stored a NULL byte in the 21st place.

Regards

Antoine.

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