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Sun May 6 08:53:13 CEST 2007 · https://mail.python.org/pipermail/python-dev/2007-May/073002.html

>> That's not a literal, it's a display. The difference is that
>> a literal denotes the same object every time it is executed.
>> A display creates a new object every time it is executed.
>> (another difference is that a display is a constructed thing
>>  which may contain runtime-computed components, unlike a
>>  literal).
>>
>> So if bytes are mutable and also have source-level
>> representation, they should be displays, not literals.
> 
> So is having mutable bytes just a matter of calling them "byte
> displays" instead of "byte literals" or does that also require
> changing something in the back end?

It's certainly also an issue of language semantics (i.e. changes
to interpreter code). There are a number of options:
1. don't support creation of byte values through syntax. Instead,
   create bytes through a constructor function.
2. if there is syntax support, make it a display: every time
   you execute a bytes display, create a new value, which
   can then be mutated.
3. if you want it to be a literal, make it immutable: change the
   type, or add a flag so that it is immutable. Then put it into
   the co_consts array of the code object.
The original complaint was that it shouldn't be in co_consts
if it is mutable.

In case these three options aren't clear yet, some examples:

1. def foo():
     return bytes([1,2,3])

   print foo() is foo() # False
   x = foo()
   x[0] = 5 # supported

2. def foo():
     return b"\x01\x02\x03"
   print foo() is foo() # False
   x = foo()
   x[0] = 5 # supported

3. def foo():
     return b"\x01\x02\x03"
   print foo() is foo() # True
   x = foo()
   x[0] = 5 # TypeError

HTH,
Martin

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[Python-Dev] Changing string constants to byte arrays in Py3k