error writing str to binary stream - fails in Python 3.0.1, works in 2.x

[John Machin]

On Mar 17, 9:29 am, "R. David Murray" <rdmur... at bitdance.com> wrote:
> walle... at gmail.com wrote:
> > On Mar 16, 4:10 pm, Benjamin Peterson <benja... at python.org> wrote:
> > >  <wallenpb <at> gmail.com> writes:
>
> > > > self.out.write(b'BM') worked beautifully.  Now I also have a similar
> > > > issue, for instance:
> > > > self.out.write("%c" % y) is also giving me the same error as the other
> > > > statement did.
> > > > I tried self.out.write(bytes("%c" %y),encoding=utf-8) in an attempt to
> > > > convert to bytes, which it did, but not binary.  How do I affect
> > > > self.out.write("%c" % y) to write out as a binary byte steam?  I also
> > > > tried self.out.write(b"%c" % y), but b was an illegal operator in when
> > > > used that way.
> > > > It is also supposed to be data being written to the .bmp file. --Bill
>
> > > Are you writing to sys.stdout? If so, use sys.stdout.buffer.write(b'some
> > > bytes'). If you're writing to a file, you have to open it in binary mode like
> > > this: open("someimage.bmp", "wb")
>
> > Yes, I am writing to a file.  That portion is correct and goes like
> > this:
>
> > self.out=open(filename,"wb")
> >     self.out.write(b"BM")          # This line works thanks to advice given
> >                                    # in previous reply
>
> > However, here is some more code that is not working and the error it
> > gives:
>
> > def write_int(self,n):
> >     str_out='%c%c%c%c' % ((n&255),(n>>8)&255,(n>>16)&255,(n>>24)&255)
> >     self.out.write(str_out)
>
> > this is line 29, does not work - not
> > sure how to get this complex str converted over to binary bytes to
> > write to bmp file.
>
> (I reformatted your message slightly to make the code block stand out more.)
>
> A byte array is an array of bytes, and it understands integers as input.
> Check out the PEP (the official docs leave some things out):
>
>    http://www.python.org/dev/peps/pep-0358/
>
> Here is some example code that works:

No it doesn't. There is an extra "(" in the assignment to bytesout.

>
>     out=open('temp', "wb")
>     out.write(b"BM")
>
>     def write_int(out, n):
>         bytesout=bytes(([n&255), (n>>8)&255, (n>>16)&255, (n>>24)&255])
>         out.write(bytesout)  
>
>     write_int(out, 125)

Consider using the struct module; it's expressly designed for that
sort of thing.

import struct
out.write(struct.pack("<2sI", "BM", 125))

HTH,
John