python - List of Structure subclass returns wrong values when casting to numpy array -


i've built simple structure subclass 2 fields, holding void pointer array, , array length. however, when try create list of these using input lists of same length, value of returned void pointer same last array used create instance:

from ctypes import pointer, c_double, c_size_t, c_void_p, structure, cast import numpy np  class external(structure):      _fields_ = [("data", c_void_p),                 ("length", c_size_t)]     @classmethod     def from_param(cls, seq):         return seq if isinstance(seq, cls) else cls(seq)      def __init__(self, seq):         self.ptr = cast(             np.array(seq, dtype=np.float64).ctypes.data_as(pointer(c_double)),             c_void_p         )         self.data = self.ptr         self.length = len(seq)         # recreate array void pointer         # shows correct values         shape = self.length, 2         ptr = cast(self.data, pointer(c_double))         array = np.ctypeslib.as_array(ptr, shape)         print "correct array", array.tolist()  if __name__ == "__main__":     interiors = [             [[3.5, 3.5], [4.4, 2.0], [2.6, 2.0], [3.5, 3.5]],             [[4.0, 3.0], [4.0, 3.2], [4.5, 3.2], [4.0, 3.0]],             ]     wrong = [external(s) s in interiors]     w in wrong:         # perform same cast array before         shape = w.length, 2         ptr = cast(w.data, pointer(c_double))         array = np.ctypeslib.as_array(ptr, shape)         print "wrong array", array.tolist()

if create external instances using input lists of difference lengths, works expected. doing wrong here?

the problem numpy array garbage-collected , underlying memory freed, resulting in dangling pointer.

the solution keep reference underlying buffer object:

def __init__(self, seq):     array = np.array(seq, dtype=np.float64)     self._buffer = array.data     self.ptr = cast(         array.ctypes.data_as(pointer(c_double)),         c_void_p     )     ...

now memory array freed when instance of external holding reference gets deleted.


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