TypeError: missing 1 required positional argument: 'self' — how to fix

Traceback (most recent call last):
  File "main.py", line 9, in <module>
    Counter.increment()
TypeError: Counter.increment() missing 1 required positional argument: 'self'

Quick fix: Instantiate the class and call the method on the object: c = Counter(); c.increment() (note the parentheses after Counter). If the method doesn't need instance data, decorate it with @staticmethod (no self) or @classmethod (takes cls).

What it means

When you call obj.method(), Python passes obj as the first argument, self. When you call ClassName.method(), there is no object, so self is missing and Python reports it like any other missing argument.

class Counter:
    def __init__(self):
        self.n = 0

    def increment(self):
        self.n += 1

Counter.increment()   # TypeError: ... missing 1 required positional argument: 'self'

Cause 1: calling the method on the class

Fix: create an instance and call the method on it.

c = Counter()
c.increment()
print(c.n)   # 1

Cause 2: forgot the parentheses when creating the instance

This is the most common version of the bug:

c = Counter       # c is the class, not an instance
c.increment()     # TypeError

Fix: c = Counter(). The same applies to callbacks and configuration, e.g. app.add_handler(MyHandler) vs app.add_handler(MyHandler()) — check what the API expects.

Cause 3: the method doesn't need an instance

If the method never uses self, make that explicit:

class MathUtil:
    @staticmethod
    def square(x):
        return x * x

MathUtil.square(4)   # 16

If it needs the class (e.g. an alternative constructor), use @classmethod:

class User:
    def __init__(self, name):
        self.name = name

    @classmethod
    def from_dict(cls, d):
        return cls(d["name"])

u = User.from_dict({"name": "ana"})

Cause 4: self missing from the method definition, or a decorator in the wrong place

If you defined a method without self and call it on an instance, the error is the reverse (takes 0 positional arguments but 1 was given). If you added @staticmethod but kept self as a parameter, calling it without arguments gives this error, because self is now an ordinary parameter. Remove self from static methods.

Cause 5: passing an unbound method as a callback

import threading
t = threading.Thread(target=Worker.run)    # Worker is the class
t.start()                                   # TypeError inside the thread

Fix: pass a bound method from an instance: w = Worker(); threading.Thread(target=w.run). The same applies to map(Cls.method, ...), sorted(key=...), button command= handlers in Tkinter, and Flask/FastAPI dependencies.

Cause 6: frameworks that expect an instance

• unittest / pytest: don't call test methods directly on the TestCase class; let the runner instantiate it.
• Django: MyView.get(request) fails; route with MyView.as_view().
• Mocking: mock.patch.object(Class, "method") replaces the function on the class; using autospec=True makes the mock require self too, so call it through an instance.

Quick check

Print what you are calling on:

print(type(c))   # <class 'type'> means you have the class, not an instance

An instance shows <class '__main__.Counter'>.

Last updated 2026-11-07.