I have two python files in one folder named firstpy. But when I am trying to import a function of python file named Sum then it gives me the error mentioned in question. The two files are:

Sum.py:

```
class Sum:
        def doSum(a,b):
        print(a+b)

```

Calculator.py:

```
from Sum import doSum

class Calculator:

    doSum(4,5)

```

Please tell me what the error is and the solutions out there are not working for me.