Hi all,
I was looking into http://bugs.python.org/issue17576 and I found that
the semantics of __int__() and __index__() are not precisely defined in
the documentation and that the implementation (CPython 3.4a) has some
odd behaviour.
Defining two classes:
class Int1(int):
def __init__(self, val=0):
print("new %s" % self.__class__)
class Int2(Int1):
def __int__(self):
return self
and two instances
i1 = Int1()
i2 = Int2()
we get the following behaviour:
>>> type(int(i1))
<class 'int'>
I would have expected 'Int1'
>>> type(int(i2))
new <class '__main__.Int2'>
<class '__main__.Int2'>
Why is a new Int2 being created?
operator.index does similar things.
So,
1. Should type(int(x)) be exactly int, or is any subclass OK?
2. Should type(index(x)) be exactly int, or is any subclass OK?
3. Should int(x) be defined as int_check(x.__int__())?
4. Should operator.index(x) be defined as index_check(x.__index__())?
where:
def int_check(x):
if is_int(x):
return x
else:
raise TypeError(...)
def index_check(x):
if is_index(x):
return x
else:
raise TypeError(...)
The definition of is_int(x) and is_index(x) follow from the answers to 1
and 2.
I had previously assumed (and would expect) that the answers were:
1. Any subclass is OK
2. Ditto
3. Yes
4. Yes
Which means that
def is_int(x):
return int in type(x).__mro__
is_index = is_int
Cheers,
Mark.
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