You get confused by concepts of function arg, default value for the arg, and member variable, you are mixing them so badly.

Function argument

For
def __init__(self,weight) you are expecting a variable passed in when calling the constructor, which is given the name “weight”. It is a function argument.

Default value for function argument

For def __init__(self,weight=0), you are still expecting a variable passed in, but you are fine if it’s absent, in which case the default value 0 will be simply assigned to symbol “weight”. This is a default value for a function argument.

Member attribute

Now for the code below:

class car:
def __init__(self, weight=0):
    self.weight = weight

It’s effectively saying, pass me a variable, I’ll assign it to a symbol “weight”, otherwise I’ll assign 0 to the symbol. And I’ll assign the value of symbol “weight” to “self.weight”, they locate at totally different scopes and hence are different symbols. It’s not like C++ when you do memberwise initialization. “self.weight” is an attribute of instances of the class.

weight vs self.weight

Hopefully that, now for the one last example, the logic will be clear enough:

class car:
    def __init__(self, weight=0):
        self.weight = 1

Here you are getting a symbol named “weight”, no matter by default or passed-in, it gets simply ignored, i.e., not referred anywhere, instead, you assign a 1 to member variable self.weight.

1

class car:
    def __init__(self, weight):
        self.weight = weight

• When initiating an object using this class, the weight argument is required (i.e.: can’t call just my_car = car()). And whatever the value of weight is passed when creating an object will be assigned to it’s attribute weight:

>>> my_car = car()
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: car.__init__() missing 1 required positional argument: 'weight'

>>> my_car = car(weight=69)
>>> print(my_car.weight)
69

2

class car:
    def __init__(self, weight):
        self.weight = 0

• When using this class to create an object, again, the weight argument is required. But no matter what the value of weight is passed when initializing an object, it’s going to be 0:

>>> my_car = car()
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: car.__init__() missing 1 required positional argument: 'weight'

>>> my_car = car(weight=69)
>>> print(my_car.weight)
0

• However, later, it is possible to change/assign a new value to weight attribute:

>>> my_car.weight = 69
>>> print(my_car.weight)
69

3

class car:
    def __init__(self, weight=0):
        self.weight = weight

• When creating an object using this class, this time the weight is an optional argument (i.e.: no need to pass, but can be passed). If weight is not passed as an argument, it will be assigned the default value, in this case 0:

>>> my_car = car()
>>> print(my_car.weight)
0
>>> my_car = car(weight=69)
>>> print(my_car.weight)
69

4

class car:
    def __init__(self, weight=0):
        self.weight = 0

• Okie, this one is interesting one (nvm).. When creating an object using this class, again, the weight argument is optional. But, what’s interesting about it is; no matter what is the value of weight that is passed as an argument or what is the default value of it, it will always going to be 0 when creating an object:

>>> my_car = car() # No errors..
>>> print(my_car.weight)
0
>>> my_car = car(weight=69) # Useless argument..
>>> print(my_car.weight)
0

5

class car:
    def __init__(self, weight=0):
        self.weight = 1

• Just like the 4 just above, weight is optional and no matter what is the default value of weight is or what is being passed as an argument when creating an object, the weight will be assigned the value 1:

>>> my_car = car()
>>> print(my_car.weight)
1
>>> my_car = car(weight=69)
>>> print(my_car.weight)
1

6

class car:
    def __init__(self):
        self.weight = 0

• This class doesn’t accept any arguments when creating an object and if you will try to pass an argument, you will get an error. And when creating an object, it’s weight attribute will be assigned value 0:

>>> my_car = car(69)
# That positional argument is `self`... 🙂
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: car.__init__() takes 1 positional argument but 2 were given

>>> my_car = car(weight=69)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: car.__init__() got an unexpected keyword argument 'weight'

>>> my_car = car()
>>> print(my_car.weight)
0

Okie, so which one is good or which one should you use? In my opinion (you can disagree), the 3rd one is good/better(?), but with slight modification:

class car:
    def __init__(self, weight):
        self.weight = weight

WHY?

1. When creating an object, you are required to pass weight as an argument or it will give error.
2. I think, no car in this world will have “weight” 0..
3. All the cars must have weight, unless it’s made out of thin air. <3

Beside that, please follow “CapWords convention” for your class names. 🙂

🫡