PEP 8: Byte Order Mark (BOM) vs coding cookie

[Terry Reedy]

twyk wrote:
> PEP 8 says ...
> 
> Files using ASCII (or UTF-8, for Python 3.0) should not have a coding
>  cookie.

> What about a BOM (Byte Order Mark)?  Per Wikipedia ...
> 
> http://en.wikipedia.org/wiki/Byte-order_mark#endnote_UTF-8)
> 
> 'In UTF-8, this is not really a "byte order" mark. It identifies the 
> text as UTF-8 but doesn't say anything about the byte order, because 
> UTF-8 does not have byte order issues.'
> 
> So is it good style to omit the BOM in UTF-8 for Python 3.0?

According to Unicode manual, yes.

http://www.unicode.org/versions/Unicode5.0.0/ch02.pdf

The endian order entry for UTF-8 in Table 2-4 is marked N/A because
UTF-8 code units are 8 bits in size, and the usual machine issues of 
endian order for larger code units do not apply. The serialized order of 
the bytes must not depart from the order defined by the UTF-
8 encoding form. Use of a BOM is neither required nor recommended for
UTF-8, but may be encountered in contexts where UTF-8 data is converted 
from other encoding forms that use a BOM or where the BOM is used as a 
UTF-8 signature. See the “Byte Order Mark” subsection in Section 16.8, 
Specials, for more information.

Since Ascii files *are*, by intentional design, UTF-8 files, and since 
Python assumes Ascii/UTF-8 as the default, in the absence of a coding 
cookie, it does not need the signature.