I created a class, something like below -

class child:
    def __init__(self,lists):
        self.myList = lists
        
    def find_mean(self):
        mean=np.mean(self.myList)
        return mean

and when I create an onject something like below -

obj=child()

it gives the error -

TypeError: __init__() missing 1 required positional argument: 'lists'

if I create object like below then it works well -

obj=child([44,22,55)

or If I create the class like below -

class child:        
    def find_mean(self,myList):
        mean=np.mean(myList)
        return mean

and then I create the object like below -

obj=child()

then also it works well, however I need to make it in the way I explained in the very begining. Can you please help me understand this context?

CodePudding user response：

In the first example, the __init__ method expects two parameters:

• self is automatically filled in by Python.
• lists is a parameter which you must give it. It will try to assign this value to a new variable called self.myList, and it won't know what value it is supposed to use if you don't give it one.

In the second example, you have not written an __init__ method. This means that Python creates its own default __init__ function which will not require any parameters. However, the find_mean method now requires you to give it a parameter instead.

When you say you want to create it in the way you explained at the beginning, this is actually impossible: the class requires a value, and you are not giving it one.

Therefore, it is hard for me to tell what you really want to do. However, one option might be that you want to create the class earlier, and then add a list to it later on. In this case, the code would look like this:

import numpy as np

class Child:
    def __init__(self, lists=None):
        self.myList = lists
        
    def find_mean(self):
        if self.myList is None:
            return np.nan
        mean = np.mean(self.myList)
        return mean

This code allows you to create the object earlier, and add a list to it later. If you try to call find_mean without giving it a list, it will simply return nan:

child = Child()
print(child.find_mean())  # Returns `nan`
child.myList = [1, 2, 3]
print(child.find_mean())  # Returns `2`

CodePudding user response：

the code you have at the top of your question defines a class called child, which has one attribute, lists, which is assigned at the time of instance creation in the __init__ method. This means that you must supply a list when creating an instance of child.

class child:
    def __init__(self, lists):
        self.myList = lists
        
    def find_mean(self):
        mean=np.mean(self.myList)
        return mean

# works because a list is provided
obj = child([44,22,55])

# does not work because no list is given
obj = child()  # TypeError

If you create the class like in your second example, __init__ is no longer being explicitly specified, and as such, the object has no attributes that must be assigned at instance creation:

class child:        
    def find_mean(self, myList):
        mean=np.mean(myList)
        return mean


# does not work because `child()` does not take any arguments
obj = child([44,22,55])  # TypeError

# works because no list is needed
obj = child()

The only way to both have the myList attribute, and not need to specify it at creation would be to assign a default value to it:

class child:        
    def find_mean(self,myList=None):
        mean=np.mean(myList)
        return mean


# now this will work
obj = child()

# as will this
obj = child([24, 35, 27])