How can I pass null to an external library, using ctypes, with an argument declared with ctypeslib.ndpointer?

If (like me) you are dealing with multiple different array types, a slightly nicer workaround is to wrap ndpointer itself:

import numpy as npfrom numpy.ctypeslib import ndpointerdef wrapped_ndptr(*args, **kwargs):  base = ndpointer(*args, **kwargs)  def from_param(cls, obj):    if obj is None:      return obj    return base.from_param(obj)  return type(base.__name__, (base,), {'from_param': classmethod(from_param)})ComplexArrayType = wrapped_ndptr(dtype=np.complex128, ndim=1, flags='C_CONTIGUOUS')DoubleArrayType = wrapped_ndptr(dtype=np.float64, ndim=1, flags='C_CONTIGUOUS')

Following the excellent advice from eryksun in a comment I subclassed the type returned by ctypeslib.ndpointer and implemented an overridden from_param. This is a little tricky as it has to be done using class factory techniques, because the class returned by ctypeslib.ndpointer is made by a factory.

The code that I ended up with:

_ComplexArrayTypeBase = numpy.ctypeslib.ndpointer(dtype=numpy.complex128, ndim=1,    flags='C_CONTIGUOUS')def _from_param(cls, obj):    if obj is None:        return obj    return _ComplexArrayTypeBase.from_param(obj)ComplexArrayType = type(    'ComplexArrayType',    (_ComplexArrayTypeBase,),    {'from_param': classmethod(_from_param)})

Many thanks to eryksun for his (as always) expert advice.