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Odd behavior when slicing with None #2508

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@cjgibson

Description

@cjgibson

Source code for slicing.pyx

def run_test(list example):
    midpoint = len(example) // 2
    print "example[midpoint:] ->", example[midpoint:]
    print "example[slice(midpoint, None)] ->", example[slice(midpoint, None)]
    print "example[midpoint:None] ->", example[midpoint:None]
    print
    endpoint = None
    print "example[slice(midpoint, endpoint)] ->", example[slice(midpoint, endpoint)]
    print "example[midpoint:endpoint] ->", example[midpoint:endpoint]

Steps to reproduce:

$ cythonize -i slicing.pyx &> /dev/null; python -c "from slicing import run_test; run_test(range(5))"
example[midpoint:] -> [2, 3, 4]
example[slice(midpoint, None)] -> [2, 3, 4]
example[midpoint:None] -> [2, 3, 4]

example[slice(midpoint, endpoint)] -> [2, 3, 4]
Traceback (most recent call last):
  File "<string>", line 1, in <module>
  File "slicing.pyx", line 9, in slicing.run_test
    print "example[midpoint:endpoint] ->", example[midpoint:endpoint]
TypeError: 'NoneType' object cannot be interpreted as an index

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