Inheritance in Python: Type Error, Wrong Number of Arguments

Question

I am trying to inherit a class constructor from a parent class. After trying several of the suggestions already on stack overflow I thought I'd ask a question myself to try and understand 1) why this code is wrong and 2) how to correct it?

The parent class:

class Submit_Copasi_Job(object):
    '''
    Submit a properly formatted copasi file to sun grid engine
    '''
    def __init__(self,copasi_file,report_name):
        self.copasi_file=copasi_file
        self.copasiML_str=self._read_copasiML_as_string()
        self.report_name=report_name
        self.submit_copasi_job_SGE()

    def _read_copasiML_as_string(self):
        '''
        Read a copasiML file as string 
        '''
        assert os.path.exists(self.copasi_file), "{} does not exist!".format(self.copasi_file)
        with open(self.copasi_file) as f:
            fle = f.read()
        return fle

        ....

The child class (which tries to use the super(SubClass, self).__init__(...) but obviously I have something wrong)

class Submit_Copasi_Multijob(Submit_Copasi_Job):
    def __init__(self):
        super(Submit_Copasi_Multijob,self).__init__(copasi_file,report_name)

    def test(self):
        return self.copasi_file

Run code

fle='D:\\MPhil\\Model_Building\\Models\\TGFB\\Fli1_Models\\M7.cps'
s=Submit_Copasi_Multijob(fle,'rep.txt')
print s.test()

All my attempts so far have resulted in a similar error:

   s=Submit_Copasi_Multijob(fle,'rep')

TypeError: __init__() takes exactly 1 argument (3 given)

Look at the __init__ method of Submit_Copasi_Multijob... where are copasi_file and report_name defined? You forgot to add the arguments. — Bakuriu
– Bakuriu, Commented Jan 24, 2016 at 13:50
@Bakuriu Yes, that was it. Thanks. If you write an answer below I'll accept. — CiaranWelsh
– CiaranWelsh, Commented Jan 24, 2016 at 13:55

timgeb · Accepted Answer · 2016-01-24 13:55:52Z

3

The problem is that the __init__ method of your child class only takes one argument, which will automatically be supplied by Python as a reference to the newly created object. So when you instantiate a Submit_Copasi_Multijob object, you cannot supply any arguments, i.e. the way your code is currently written the correct way to instantiate an object of your child class is

new_obj = Submit_Copasi_Multijob()

Of course, this will fail while executing __init__ because the names copasi_file and report_name won't be defined.

Fix this by letting the __init__ method of your child-class take three arguments instead of one:

def __init__(self, compasi_file, report_name):
    super(Submit_Copasi_Multijob,self).__init__(copasi_file,report_name)
    # other stuff

However, if this is the only thing you ever plan to do in the __init__ method of your child-class (i.e. there's no # other stuff), there's no need to override the __init__ method of your base class. In that case, just omit the definition of __init__ in Submit_Copasi_Multijob.

answered Jan 24, 2016 at 13:55

timgeb

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2 Comments

CiaranWelsh Over a year ago

Hi timgeb, just to clarify, if I only had the copasi_file and report_name arguments in my subclass I do not need a constructor at all?

timgeb Over a year ago

@user3059024 yes, depending on what you want to do. If you omit the definition of __init__ in your sublass, it will inherit the __init__ from the base class. If the __init__ method of the subclass is supposed to do exactly the same thing as the __init__ method of the base class, then there's no need to re-define __init__ in the subclass.

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