Two Class instances in Python not different

Question

I'm working on another data acquisition project, which has turned into an object oriented programming question. In “main” at the bottom of my code I make two instances of the Object DAQInput. When I wrote this, I thought my method .getData would refer to the taskHandle of the particular instance, but it does not. When I run, the code does the getData task with the first handle twice, so clearly I don’t really understand object oriented programming in Python. I’m sorry this code will not run without PyDAQmx and a National Instruments board attached.

from PyDAQmx import *
import numpy

class DAQInput:
    # Declare variables passed by reference
    taskHandle = TaskHandle()
    read = int32()
    data = numpy.zeros((10000,),dtype=numpy.float64)
    sumi = [0,0,0,0,0,0,0,0,0,0]

    def __init__(self, num_data, num_chan, channel, high, low):
        """ This is init function that opens the channel"""
        #Get the passed variables
        self.num_data = num_data
        self.channel = channel
        self.high = high
        self.low = low
        self.num_chan = num_chan

        # Create a task and configure a channel
        DAQmxCreateTask(b"",byref(self.taskHandle))
        DAQmxCreateAIThrmcplChan(self.taskHandle, self.channel, b"",
                                 self.low, self.high,
                                 DAQmx_Val_DegC,
                                 DAQmx_Val_J_Type_TC,
                                 DAQmx_Val_BuiltIn, 0, None)
        # Start the task
        DAQmxStartTask(self.taskHandle)

    def getData(self):
        """ This function gets the data from the board and calculates the average"""
        print(self.taskHandle)
        DAQmxReadAnalogF64(self.taskHandle, self.num_data, 10,
                           DAQmx_Val_GroupByChannel, self.data, 10000,
                           byref(self.read), None)

        # Calculate the average of the values in data (could be several channels)
        i = self.read.value
        for j in range(self.num_chan):
            self.sumi[j] = numpy.sum(self.data[j*i:(j+1)*i])/self.read.value

        return self.sumi

    def killTask(self):
        """ This function kills the tasks"""
        # If the task is still alive kill it
        if self.taskHandle != 0:
            DAQmxStopTask(self.taskHandle)
            DAQmxClearTask(self.taskHandle)

if __name__ == '__main__':
    myDaq1 = DAQInput(1, 4, b"cDAQ1Mod1/ai0:3", 200.0, 10.0)
    myDaq2 = DAQInput(1, 4, b"cDAQ1Mod2/ai0:3", 200.0, 10.0)
    result = myDaq1.getData()
    print (result[0:4])

    result2 = myDaq2.getData()
    print (result2[0:4])

    myDaq1.killTask()
    myDaq2.killTask()

Chances are that your shared objects (in class variables, e.g. `sumi`) are being munged (but I quickly stopped looking as the case is not minimal and the *symptoms* and expected behavior not been clearly described). See [Variables inside and outside of a class __init__() function](http://stackoverflow.com/questions/1537202/variables-inside-and-outside-of-a-class-init-function) — user2246674, Apr 26 '13 at 19:50

score 3 · Accepted Answer · answered Apr 26 '13 at 19:54

These variables:

class DAQInput:
    # Declare variables passed by reference
    taskHandle = TaskHandle()
    read = int32()
    data = numpy.zeros((10000,),dtype=numpy.float64)
    sumi = [0,0,0,0,0,0,0,0,0,0]

Are class variables. They belong to the class itself and are shared among instances of the class (i.e. if you modify self.data in Instance1, Instace2's self.data is modified as well).

If you want them to be instance variables, define them in __init__.

Two Class instances in Python not different

1 Answers1