Python : Passing arguments during instantiation

Question

I have a class called "data-packet" shown below. I am trying to instantiate it so that I can use the values returned by the methods getAcc and getRot. I have also included the code that is creating an instance of the data_packet class in "main" I am getting this error:

Traceback (most recent call last)
mydata = data_packet()
TypeError: __init__() takes exactly 2 arguments (1 given)

My assumption is that the instance is passing mydata to the argument called self, but nothing is being passed to l. If I am correct, how can I solve this error?

class data_packet (object):
    def __init__(self, l):

        self.data = [0,0,0,0,0,0,0,0,0]
        self.type = ord(l[2])

        if self.type == 0:   # accel

            self.data[0] = four_bytes(l[3],l[4],l[5],l[6]) * 1.0 / (1<<16)
            self.data[1] = four_bytes(l[7],l[8],l[9],l[10]) * 1.0 / (1<<16)
            self.data[2] = four_bytes(l[11],l[12],l[13],l[14]) * 1.0 /(1<<16)

        elif self.type == 6:   # heading
            self.data[0] = four_bytes(l[3],l[4],l[5],l[6]) * 1.0 / (1<<16)

        else:   # unsupported
            pass


    def display(self):    
        if self.type == 0:
            print 'accel: %7.3f %7.3f %7.3f' % \
                     (self.data[0], self.data[1], self.data[2])

        elif self.type == 6:
            print 'heading: %7.4f' % self.data[0]

        else:
            print 'what?'

    def getAcc(self):
        while self.type == 0:

            .....dosomethings....

        return accelData

    def getRot(self):
        while self.type == 5:

        ..... dosomethings....

        return rotData


def main():
    mydata = data_packet()
    mydata.getAcc()
    mydata.getRot()
    print "********* : ", mydata.getAcc()


if __name__ == "__main__": 
    main()

You say the code is below, but you didnt actually put any of the code. That would help us a lot — dwanderson
– dwanderson, Commented Dec 16, 2015 at 13:40
My apologies... I have been struggling to post the code. Check the edited version of the question. Thanks — Burgs
– Burgs, Commented Dec 16, 2015 at 13:47
Er, you solve this by passing the value of l in the instantiation. Why is that confusing? — Daniel Roseman
– Daniel Roseman, Commented Dec 16, 2015 at 13:48
@a_bhi_9 : actually it has to be subscriptable, not iterable... — bruno desthuilliers
– bruno desthuilliers, Commented Dec 16, 2015 at 13:53

Marc Schulder · Accepted Answer · 2015-12-16 13:58:46Z

1

The self in object methods is a reference to the instantiated object. When calling a method via dot notation, e.g. mydata.getAcc(), the object is automatically passed on as the first parameter. A more explicit way of achieving the same is calling the method via the class: data_package.getAcc(mydata).

__init__ is called automatically at object instantiation, but otherwise it works the same. Writing data_package() can be imagined as data_package.__init(new_object). However, in your class definition, you defined a second parameter l for __init. Your object instantiation should be data_package(l), that way, l is passed over to __init__ as its second parameter. i.e. data_package.__init(new_object, l).

answered Dec 16, 2015 at 13:58

Marc Schulder

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11 Comments

Burgs Over a year ago

When I pass "l" as the argument as follows mydata = data_packet(l), I get this NameError: global name 'l' is not defined

Burgs Over a year ago

Thank you all for the feedback

toti08 Over a year ago

You have to define l before passing it to your class instantiation. Most probably l is a list...

Burgs Over a year ago

Any suggestions on how I can do that given the above code??

Marc Schulder Over a year ago

well, where is the data coming from that you want to put into four_bytes()?

|

bruno desthuilliers · Accepted Answer · 2015-12-16 13:51:22Z

1

The answer seems pretty obvious from both the code and the traceback: your data_packet class takes a required argument l (that is "L" lower-case - an identifier one should never use...) - obviously a list FWIW - that you don't pass.

NB:self is the current instance and is automagically passed in.

answered Dec 16, 2015 at 13:51

bruno desthuilliers

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2 Comments

Burgs Over a year ago

When I pass "l" as the argument as follows mydata = data_packet(l), I get this NameError: global name 'l' is not defined

Βασιλης Ιωσηφιδης Over a year ago

l is not defined because you have probably not initialized it before creating the data_packet object. also you do not have to name it l when you pass it to your object i.e mydata = data_packet([1,2,3,4,5,6,7,8,9,....])

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