TypeError: method() takes 1 positional argument but 2 were given

Question

If I have a class...

class MyClass:      def method(arg):         print(arg) 

...which I use to create an object...

my_object = MyClass() 

...on which I call method("foo") like so...

>>> my_object.method("foo") Traceback (most recent call last): File "<stdin>", line 1, in <module> TypeError: method() takes exactly 1 positional argument (2 given) 

...why does Python tell me I gave it two arguments, when I only gave one?

Answer 1

In Python, this:

my_object.method("foo") 

...is syntactic sugar, which the interpreter translates behind the scenes into:

MyClass.method(my_object, "foo") 

...which, as you can see, does indeed have two arguments - it's just that the first one is implicit, from the point of view of the caller.

This is because most methods do some work with the object they're called on, so there needs to be some way for that object to be referred to inside the method. By convention, this first argument is called self inside the method definition:

class MyNewClass:      def method(self, arg):         print(self)         print(arg) 

If you call method("foo") on an instance of MyNewClass, it works as expected:

>>> my_new_object = MyNewClass() >>> my_new_object.method("foo") <__main__.MyNewClass object at 0x29045d0> foo 

Occasionally (but not often), you really don't care about the object that your method is bound to, and in that circumstance, you can decorate the method with the builtin staticmethod() function to say so:

class MyOtherClass:      @staticmethod     def method(arg):         print(arg) 

...in which case you don't need to add a self argument to the method definition, and it still works:

>>> my_other_object = MyOtherClass() >>> my_other_object.method("foo") foo 

Answer 2

In simple words.

In Python you should add self argument as the first argument to all defined methods in classes:

class MyClass:   def method(self, arg):     print(arg) 

Then you can use your method according to your intuition:

>>> my_object = MyClass() >>> my_object.method("foo") foo 

This should solve your problem :)

For a better understanding, you can also read the answers to this question: What is the purpose of self?