TypeError: Class() takes no arguments

This post is about one of the type errors that occur when dealing with __init__ in Python classes.

Table of Contents

For more information about argument types and parameter types, see Argument Types in Python Functions, Parameter Types in Python Functions.

What does the error message mean?

This type of error can occur when we pass some argument values to the __init__ method of a class while initializing an object.

    class_instance = MyClass(<some arguments here>)
TypeError:  MyClass() takes no arguments

In versions of Python 3.6 and earlier, the error message may look different.

TypeError: object() takes no parameters

This error means that there is no initialization method defined in the class, or the method name is misspelled. In both cases, we cannot pass the arguments required for a new instance of the class.

If we want to pass arguments, we need to add a method and specify the parameters in that method.

If `init` is spelled incorrectly

The most common errors in the method name are missing underscores, missing letters, or rearranged letters.

_init_, __init_, __int__, __inti__

__init__ is a special method used to customize an instance of a class. If there is a typo in the name of a method, it is not treated as a special method, but as a normal method of the class.

Class with the correct `init` method

class A:
    def __init__(self, x, y):
        self.x = x
        self.y = y

# passing arguments without errors
a = A(1, 2)

We will get an error if we forget to pass arguments to the method.

a = A()
# TypeError: A.__init__() missing 2 required positional arguments: 'x' and 'y'

Class with the incorrect `_init_` method

class A:
    def _init_(self, x, y):
        self.x = x
        self.y = y
        
# no missing arguments 'x' and 'y'
a = A()

# _init_ is a normal method
a._init_(1, 2)
print(a.x, a.y)
# 1 2

We will get an error if we try to pass arguments to a method.

a = A(1, 2)
# TypeError: A() takes no arguments

If `init` is not defined in the class

Because most Python 3 classes inherit methods from the base object class, we don't get errors if our class omits an __init__ method.

class A:
    pass

# no errors if we pass no values
a = A()

print(issubclass(A, object))
# True

# method resolution order for class A
# If __init__ is not found in class A, it is looked for in the next parent class.
print(A.__mro__)
# (<class '__main__.A'>, <class 'object'>)

Calling object() returns a new featureless object. When an object.__init__ is called, it does not need to accept any arguments intended to customize an instance of the class. This raises a TypeError. When our class does not have an __init__ method defined, it behaves just like the parent class.

>>> obj = object("value")
Traceback (most recent call last):
  File "<pyshell#0>", line 1, in <module>
    obj = object("value")
TypeError: object() takes no arguments
>>> class A:
        pass

>>> a = A("value")
Traceback (most recent call last):
  File "<pyshell#4>", line 1, in <module>
    a = A("value")
TypeError: A() takes no arguments

As we can see, our class uses the parent __init__.

>>> object.__init__ is A.__init__
True

We have to redefine the __init__ method in our class with the same name.

NB