Terry J. Reedy added the comment:
The library doc says that if the source argument for bytes() or bytearray() is
a string or integer it is treated a certain way. I believe 'string' and
'integer' are meant to include subclasses of str and int, else it should have
said str or int. You have verified that int subclass instances are indeed
treated as integers. The following shows that str subclass instances are
treated as strings.
class mystr(str):
def __bytes__(self): return b'hello'
print(bytes(mystr('a')))
raises TypeError: string argument without an encoding
The language reference says "object.__bytes__(self): Called by bytes() to
compute a byte-string representation of an object." No exception is given. I
see this as a contradiction in the manual, in which case the current behavior
rules and the contradiction is removed by changing the doc part that conflicts
with behavior. That would mean extending the doc sentence with 'that is not a
string or integer'.
However, I am waiting for other opinions for two reasons.
1. The doc also specified special treatment for buffer interface and iterable
objects. However, __bytes__ *is* checked first for those objects.
class myit(list):
def __bytes__(self): return b'hello'
print (bytes (myit([1,2,3])))
# b'hello'
class mybit(bytes): # example
def __bytes__(self): return b'hello'
print (bytes (mybit(b'a')))
# b'hello'
So the exception is limited to 'strings' and 'integers', making it somewhat
inconsistent.
2. If someone gives an str or int subclass a __bytes__ method, they can only
mean for it to be called by bytes(), as there is no other use of that special
method.
On the third hand, a user of a class who tests its instances before calling
bytes with "isinstance(x, (str, int))" instead of "type(x) is str or type(x) is
int", likely expects getting normal string or integer behavior. Of course, the
same might be true if the test is 'isiterable(x)'.
If behavior were changed, I think it should wait for 3.4 since the current
behavior is not clearly a bug to me.
----------
assignee: -> docs@python
components: +Documentation -Interpreter Core
nosy: +docs@python, terry.reedy
title: __bytes__ doesn't work in subclass of int -> __bytes__ doesn't work in
subclass of int and str
versions: +Python 2.7, Python 3.3, Python 3.4
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<http://bugs.python.org/issue17309>
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