what does := means simply?

[MRAB]

On 2018-05-19 13:28, bartc wrote:
> On 19/05/2018 12:38, Chris Angelico wrote:
>> On Sat, May 19, 2018 at 8:33 PM, bartc <bc at freeuk.com> wrote:
> 
>>> But then you are acknowledging the file is, in fact, ASCII.
>> 
>> Cool! So what happens if you acknowledge that a file is ASCII, and
>> then it starts with a byte value of E3 ?
> 
> It depends.
> 
> If this is a .ppm file I'm trying to read, and it starts with anything
> other than 'P' followed by one of '1','2','3','4','5','6' (by which I
> mean the ASCII codes for those), then it's a bad ppm file.
> 
> What are you really trying to say here?
> 
> Out of interest, how would Python handle the headers for binary file
> formats P4, P5, P6? I'd have a go but I don't want to waste half the day
> trying to get past the language.
> 
> It is quite possible to deal with files, including files which are
> completely or partially text, a byte at a time, without having silly
> restrictions put on them by the language.
> 
> Here's the palaver I had to go through last time I wrote such a file
> using Python, and this is just for the header:
> 
>       s="P6\n%d %d\n255\n" % (hdr.width, hdr.height)
>       sbytes=array.array('B',list(map(ord,s)))
>       f.write(sbytes)
> 
> Was there a simple way to write way to do this? Most likely, but you
> have to find it first! Here's how I write it elsewhere:
> 
>     println @f, "P6"
>     println @f, width,height
>     println @f, 255
> 
> It's simpler because it doesn't get tied up in knots in trying to make
> text different from bytes, bytearrays or array.arrays.
> 
It's very simple:

     s = b"P6\n%d %d\n255\n" % (hdr.width, hdr.height)
     f.write(s)