The Cython code obj[i] where i is a cdef int is translated to a call to __Pyx_GetItemInt_Fast.
This will use the sequence protocol to get obj[i]. More precisely, Py_TYPE(obj)->tp_as_sequence->sq_item(obj, i) will be called.
However, Python always tries the mapping protocol first. Because of this, it can happen that obj[i] in Cython behaves differently from Python. There is nothing in the Python docs that says that the mapping and sequence protocols should agree, so this is a bug.
Simple repro:
def get(obj, int i):
return obj[i]
class D(dict):
pass
This gives:
>>> d = D([(-1, "hello")]); d
{-1: 'hello'}
>>> d[-1]
'hello'
>>> get(d, -1)
KeyError: 0
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