How should methods be created for a class that inherits from NumPy ndarray?

Question

I'm creating a class that inherits from a NumPy ndarray. I'm having a bit of trouble giving it methods. Specifically, when I add a simple method printout to the class, I get the following error:

AttributeError: 'NoneType' object has no attribute 'printout'

How should I be adding methods to this class? Also, preemptively, is there a recommended way to change the value of self internally in this class?

import numpy

class Variable(numpy.ndarray):

    def __new__(
        cls,
        name                    = "trk_pt",
        tree                    = None, # tree object
        eventNumber             = None,
        eventWeight             = None,
        numberOfBins            = None, # binning
        binningLogicSystem      = None, # binning
        ):
        self = numpy.asarray([]).view(cls)
        # arguments
        self._name              = name
        self.tree               = tree
        self.eventNumber        = eventNumber
        self.eventWeight        = eventWeight
        self.numberOfBins       = numberOfBins
        self.binningLogicSystem = binningLogicSystem
        # internal
        self.variableObject     = None
        self.variableType       = None
        self.dataType           = None
        self.variableDataTypes  = None
        self.canvas             = None
        self.histogram          = None
        self._values            = [] # list of values
        self._valuesRaw         = [] # list of unmodified, raw values

    def printout(
        self
        ):
        print("hello world")

a = Variable()
a.printout()

If you're using __new__, you have to return the newly created instance of your class. — sebastian
– sebastian, Commented Dec 19, 2014 at 16:42

ProgrammingIsAwsome · Accepted Answer · 2014-12-19 16:52:46Z

2

As @sebastian said, new must return the instance you create. Just add return self at the end of the constructor (I've tested it and it worked for me):

   ...
   self._values            = [] # list of values
   self._valuesRaw         = [] # list of unmodified, raw values

   return self

answered Dec 19, 2014 at 16:52

ProgrammingIsAwsome

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1 Comment

d3pd Over a year ago

Great, thanks very much for your help with that. Inheriting from NumPy arrays is certainly a bit different from usual Python inheriting.

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