Implement Bytes.__new__

[BCSharp]

This is primarily to support the following case (from stdlib tests):

class BytesSubclass(bytes):
    pass

class SomeClass:
    def __bytes__(self):
        return BytesSubclass(b'abc')

assert type(bytes(SomeClas())) is BytesSubclass

Also an opportunity to clean up constructor overloads in Bytes.

[slozier]

Seems generally good.

Not that I necessarily think this PR needs to be amended for these cases (since we weren't supporting them before), but we're missing the "always call __bytes__" whenever we hit specialized __new__ overloads:

class BytesSubclass(bytes):
  def __bytes__(self):
    return BytesSubclass(b"x")

x = bytes(BytesSubclass(b"y"))
assert type(x) is BytesSubclass and x == b"x"

class Test(str):
  def __bytes__(self):
    return b"bytes"

assert Test("str") == b"bytes"

[slozier]

Calling __new__ from C# seems a bit icky. I wonder if we should have a Bytes Bytes.FromObject(object) helper instead.

[BCSharp]

Yeah, I wasn't fond of it either. Will try to factor it out.

[BCSharp]

Oh, I forgot about __bytes__ from bytes... I tested it with other types though (list, str). I intended this PR to get the bytes construction right so I will amend it.

assert Test("str") == b"bytes"

I assume you meant

assert bytes(Test("str")) == b"bytes"

This one throws on CPython 3.4 (which is the one I use for testing for IronPython 3) but now I see it works under CPython 3.5. It looks to me like an ommision in CPython 3.4 that got rectified in the next release so I am going to implement the later behaviour.

[slozier]

Looks good to me assuming the tests pass. Just a minor suggestion.

[slozier]

Maybe an if (source.GetType() == typeof(Bytes)) return source;? Could use the same optimization in FromObject.

[BCSharp]

How about a separate overload?

[slozier]

What sort of overload?

[BCSharp]

In 05ceb50 I added an overload for the C# constructor with a Bytes parameter as a fast track. I was also thinking about an overload for the Python __new__ constructor, but this would not work: it would catch also the subclasses and those have to be tried with __bytes__.