Class issue python

Question

I'm trying to learn more about objects, and from what I understood, self doesn't take any value. Why does this method asks for another argument when I try to call it? Thanks.

class School:
        def __init__(self):
            self.roster =  []
            self.dicti = {1:[],
                         2:[],
                         3:[],
                         4:[],
                         5:[]}
                         
        def add_student(self,name,grade):
            if name in self.dicti[grade]:
                raise ValueError("The student is already added to the grade")
            else:
                self.dicti[grade].append(name)
            
    
    
            
    x = School
    print(x.add_student("radu",2))

sarema · Accepted Answer · 2022-01-13 12:02:49Z

2

You need to instantiate the class. In your penultimate line:

x = School()

Without the paranthesis, x is just a reference to School. You need to "call" School() to construct and initialise an object and assign it to x.

What is happening under the hood is that when you have an object of the class, self is automatically replaced with a reference to the object. Without instantiating the class, this cannot be done, therefore python asks you to provide another argument to take the position of self.

answered Jan 13, 2022 at 12:02

sarema

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Another Noob · Accepted Answer · 2022-01-13 12:17:09Z

You need to initiate the class as Sarema wrote. You also don't return anything from the add_student function, so the print will write None

class School:
    def __init__(self):
        self.roster = []
        self.dicti = {1: [],
                      2: [],
                      3: [],
                      4: [],
                      5: []}

    def add_student(self, name, grade):
        if name in self.dicti[grade]:
            raise ValueError("The student is already added to the grade")
        else:
            self.dicti[grade].append(name)
            return f"Added {name} to list grade: {grade}. " \
                   f"\nGrade {grade} list: {self.dicti[grade]}"


x = School()
print(x.add_student("radu", 2))

returns

Added radu to list grade: 2. 
Grade 2 list: ['radu']

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