This came up in Dask (dask/dask#4387). We have an object (dask.delayed.Delayed) that duck-types method/attribute access to build a graph from user code. Unfortunately this means that when np.dtype tries to get the dtype attribute it succeeds but doesn't return a valid dtype (it returns another delayed object). This leads to recursion and eventually a segfault.

I'm able to reproduce his without dask as follows:

import numpy as np

class ShouldHaveNoDtype(object):
    """This class happens to have a dtype attribute, but it doesn't actually
    meet the interface"""
    @property
    def dtype(self):
        """Has the attribute, but doesn't return a dtype, leading to recursion"""
        return self


x = ShouldHaveNoDtype()
np.dtype(x)