bytes should implement __new__

This seems like a similar issue to #2091. The bytes type implements __init__ but doesn't appear to provide an overload for __new__. This makes it harder for subclasses of bytes to define their own behaviour for __new__.

typeshed/stdlib/3/builtins.pyi

Lines 344 to 355 in 9c3978e

    
           class bytes(ByteString): 
        
               @overload 
        
               def __init__(self, ints: Iterable[int]) -> None: ... 
        
               @overload 
        
               def __init__(self, string: str, encoding: str, 
        
                            errors: str = ...) -> None: ... 
        
               @overload 
        
               def __init__(self, length: int) -> None: ... 
        
               @overload 
        
               def __init__(self) -> None: ... 
        
               @overload 
        
               def __init__(self, o: SupportsBytes) -> None: ...

This also appears to be the case for some of the other builtins (e.g. str)

typeshed/stdlib/3/builtins.pyi

Lines 267 to 272 in 9c3978e

    
           class str(Sequence[str]): 
        
               @overload 
        
               def __init__(self, o: object = ...) -> None: ... 
        
               @overload 
        
               def __init__(self, o: bytes, encoding: str = ..., errors: str = ...) -> None: ...

Could you give an example where this is a problem? bytes and str should inherit __new__ from object if I am not mistaken.

They do, but the argument list is wrong. As a strawman example:

class SHA1Hash(bytes):
    def __new__(cls, b: bytes) -> 'SHA1Hash':
        if len(b) != 20:
            raise ValueError("Invalid SHA1Hash value!")
        return super().__new__(cls, b)

This doesn't typecheck, because the call to super().__new__(cls, b) is inferred to be object.__new__(cls, b) which is incorrect as object.__new__ takes only the cls argument. bytes, on the other hand, takes an argument to create the object.

This is also a bit like the tuple case again in that __init__ doesn't make much sense for bytes or str, as they are non-mutable and can only be constructed with __new__ in the first place.

Thanks for the explanation. Makes sense. We appreciate pull requests!

str.__new__ had been added in #1352 but then reverted

Figure I should add a ref to the reason it was backed out: #1464

It appears it was backed out because the addition of __new__ to str caused problems for classes deriving from both str and Enum, as the str's __new__ would now take priority rather than the Enum's. I'm guessing this is caused by some magic wrt how str is actually implemented within python :-/

Perhaps there needs to be a special case for these builtins to make constructing them work correctly?

Use __new__ instead of __init__ for some builtin classes

Use __new__ instead of __init__ for some builtin classes (#4555)

Closed by #4555 (and thank you for git-revise!)

performance: remove redundant cast

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`bytes` should implement `new` #2630

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	class bytes(ByteString):
	@overload
	def __init__(self, ints: Iterable[int]) -> None: ...
	@overload
	def __init__(self, string: str, encoding: str,
	errors: str = ...) -> None: ...
	@overload
	def __init__(self, length: int) -> None: ...
	@overload
	def __init__(self) -> None: ...
	@overload
	def __init__(self, o: SupportsBytes) -> None: ...

	class str(Sequence[str]):
	@overload
	def __init__(self, o: object = ...) -> None: ...
	@overload
	def __init__(self, o: bytes, encoding: str = ..., errors: str = ...) -> None: ...

bytes should implement __new__ #2630

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srittau commented on Nov 29, 2018

mystor commented on Nov 29, 2018

srittau commented on Nov 29, 2018

sproshev commented on Nov 30, 2018

mystor commented on Nov 30, 2018

hauntsaninja commented on Sep 30, 2020

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`bytes` should implement `new` #2630