CODEWITHSUNDEEP
pythonpython-2.7python-internals
Jonathon Reinhart
Jonathon Reinhart

Reputation: 137438

Slice indices limited to 0x7FFFFFFF

I'm playing with slices in Python (2.7.4):

class Foo():
    def __getitem__(self, key):
        # Single value
        if isinstance(key, (int,long)):
            return key

        # Slice
        if isinstance(key, slice):
            print 'key.start = 0x{0:X}   key.stop = 0x{1:X}'.format(key.start, key.stop)
            length = key.stop - key.start
            return str(length)

Everything seems to work as expected:

>>> f = Foo()
>>>
>>> f[42]
42
>>>
>>> f[20:30]
key.start = 0x14   key.stop = 0x1E
'10'

Except it seems that the slice indices are limited to 0x7FFFFFFF:

>>> f[0xFEDCBA98 : 0xFFFFFFFF]
key.start = 0x7FFFFFFF   key.stop = 0x7FFFFFFF
'0'
>>> f[0x80000000:0x90000000]
key.start = 0x7FFFFFFF   key.stop = 0x7FFFFFFF
'0'

Why are slice indices not subject to the same long integer promotion as regular int values? Is there any workaround for this?

Upvotes: 8

Views: 379

Answers (2)

thefourtheye
thefourtheye

Reputation: 239483

After a lot of searching I found this

In python 3.3, the slice start and end are found like this

start = PyLong_FromSsize_t(istart);
...
end = PyLong_FromSsize_t(istop);

but in 2.7 they are found like this

start = PyInt_FromSsize_t(istart);
...
end = PyInt_FromSsize_t(istop);

In 2.7, PyInt_FromSsize_t ultimately uses the size of long where as in 3.3, PyLong_FromSsize_t uses the size of PyObject. That is why it works fine in 3.3 and not in 2.7.

Upvotes: 7

Jonathon Reinhart
Jonathon Reinhart

Reputation: 137438

I've realized that this appears to be a limitation of old-style classes. New-style classes (ones that derive from object) behave as expected:

class Foo(object):
   #...

Results:

>>> f = Foo()
>>>
>>> f[0x80000000:0x90000000]
key.start = 0x80000000   key.stop = 0x90000000
'268435456'
>>>
>>> f[0xFEDCBA98 : 0x1200000000]
key.start = 0xFEDCBA98   key.stop = 0x1200000000
'73033532776'
>>>

I haven't seen this documented anywhere. It's especially confusing, since this limitation is not in the slice class itself:

>>> s = slice(0xFEDCBA98, 0x1200000000)
>>>
>>> s
slice(4275878552L, 77309411328L, None)
>>>
>>> hex(s.start)
'0xfedcba98L'

Upvotes: 7

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