[Python-Dev] Pre-PEP: The "bytes" object

Guido van Rossum guido at python.org
Thu Feb 16 21:47:22 CET 2006

On 2/15/06, Neil Schemenauer <nas at arctrix.com> wrote:
> This could be a replacement for PEP 332.  At least I hope it can
> serve to summarize the previous discussion and help focus on the
> currently undecided issues.
> I'm too tired to dig up the rules for assigning it a PEP number.
> Also, there are probably silly typos, etc.   Sorry.

I may check it in for you, although right now it would be good if we
had some more feedback.

I noticed one behavior in your pseudo-code constructor that seems
questionable: while in the Q&A section you explain why the encoding is
ignored when the argument is a str instance, in fact you require an
encoding (and one that's not "ascii") if the str instance contains any
non-ASCII bytes. So bytes("\xff") would fail, but bytes("\xff",
"blah") would succeed. I think that's a bit strange -- if you ignore
the encoding, you should always ignore it. So IMO bytes("\xff") and
bytes("\xff", "ascii") should both return the same as bytes([255]).
Also, there's a code path where the initializer is a unicode instance
and its encode() method is called with None as the argument. I think
both could be fixed by setting the encoding to
sys.getdefaultencoding() if it is None and the argument is a unicode

    def bytes(initialiser=[], encoding=None):
        if isinstance(initialiser, basestring):
            if isinstance(initialiser, unicode):
                if encoding is None:
                    encoding = sys.getdefaultencoding()
                initialiser = initialiser.encode(encoding)
            initialiser = [ord(c) for c in initialiser]
        elif encoding is not None:
            raise TypeError("explicit encoding invalid for non-string "
        create bytes object and fill with integers from initialiser
        return bytes object

BTW, for folks who want to experiment, it's quite simple to create a
working bytes implementation by inheriting from array.array. Here's a
quick draft (which only takes str instance arguments):

    from array import array
    class bytes(array):
        def __new__(cls, data=None):
            b = array.__new__(cls, "B")
            if data is not None:
            return b
        def __str__(self):
            return self.tostring()
        def __repr__(self):
            return "bytes(%s)" % repr(list(self))
        def __add__(self, other):
            if isinstance(other, array):
                return bytes(super(bytes, self).__add__(other))
            return NotImplemented

--Guido van Rossum (home page: http://www.python.org/~guido/)

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