Class variables - missing one required positional argument

Question

I have two scripts. The first containing a class, with class variables defined and a function using those class variables. The second script calls the class and function within a function of it's own.

This sort of set up works fine for functions inside a class, however adding class variables is causing me the below error. Can anyone explain why, please and what I need to do to fix?

Thanks

obj1.py:

class my_test_class():

    def __init__(self):

        self.test1 = 'test1'
        self.test2 = 'test2'
        self.test3 = 'test3'

    def test_func(self, var):

        new_var = print(var, self.test1, self.test2, self.test3)

obj2.py

from obj1 import *


def import_test():

    target_var = my_test_class.test_func('my test is:')
    print(target_var)

import_test()

Error:

Traceback (most recent call last):
  File "G:/Python27/Test/obj2.py", line 9, in <module>
    import_test()
  File "G:/Python27/Test/obj2.py", line 6, in import_test
    target_var = my_test_class.test_func('my test is:')
TypeError: test_func() missing 1 required positional argument: 'var'

You didn't create an instance of my_test_class before trying to call an instance method (since test_func expects self) — roganjosh
– roganjosh, Commented May 11, 2019 at 12:08
Separately, my_test_class should be MyTestClass and you should avoid * imports. See PEP8. This convention is actually really important for others to understand your code and avoid bugs. — roganjosh
– roganjosh, Commented May 11, 2019 at 12:09
Create an instance test_class = my_test_class() and then call test_class.test_func('my test is:') — vb_rises
– vb_rises, Commented May 11, 2019 at 12:10
hi. thanks all. yes, that fixed my problem. always get a bit confused with this issue... — gdogg371
– gdogg371, Commented May 11, 2019 at 12:17
Hi @gdogg371, I have tried to put the comments from above into an answer, please take a look below :) — Devesh Kumar Singh
– Devesh Kumar Singh, Commented May 11, 2019 at 14:07

Devesh Kumar Singh · Accepted Answer · 2019-05-11 14:06:20Z

As the commentors have pointed out, since the test_func is a class method, we need to call it using a class instance object.

Also print function returns None, so doing new_var = print(var, self.test1, self.test2, self.test3) assigns new_var=None, so if you want to return the variable, you need to assign new_var = ' '.join([var, self.test1, self.test2, self.test3]), which creates a string with a whitespace between all the words, and return new_var

Combining all of this, the code comes out as follows

class my_test_class():

    def __init__(self):

        self.test1 = 'test1'
        self.test2 = 'test2'
        self.test3 = 'test3'

    def test_func(self, var):

        #Assign value to new_var and return it
        new_var = ' '.join([var, self.test1, self.test2, self.test3])
        return new_var

def import_test():

    #Create instance of my_test_class
    test_class = my_test_class()
    #Call test_func using instance of my_test_class
    print(test_class.test_func('my test is:'))

import_test()

The output will be my test is: test1 test2 test3

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