Answer.

Python supports three types of methods in class definitions: instance methods, class methods, and static methods. They differ in how they are called and their access to class and instance data.

History:

Initially, instance methods (those with "self") were the only type of behavior; it was implied that methods always invoke behavior or read/change data of a specific object. Later, class methods (with "cls") were introduced in Python, providing behavior for the class as a whole (e.g., alternative constructors), and static methods, which are similar to regular functions but associated with the class.

Problem:

Sometimes common functionality is required for all instances (static methods). Other times, an operation should be available "for the entire class" (e.g., creating an instance). If the method type is incorrectly defined, bugs may arise (for example, accidentally trying to modify the class through an instance method or vice versa).

Solution:

Instance methods (regular methods, self parameter): Access to data and behavior of a specific object.
Class methods (with the @classmethod decorator, cls parameter): Access to the state of the class but not to the state of the object. Used for creating alternative constructors and for operations that should pertain to the class and not the instance.
Static methods (with the @staticmethod decorator): Do not have access to either self or cls; regular functions within the class namespace.

Code example:

class MyClass:
    def instance_method(self):
        return f"Instance: {self}"

    @classmethod
    def class_method(cls):
        return f"Class: {cls}"

    @staticmethod
    def static_method():
        return "This is a static method"

obj = MyClass()
print(obj.instance_method())  # Instance: <MyClass object...>
print(MyClass.class_method()) # Class: <class 'MyClass' >
print(MyClass.static_method())# This is a static method

Key features:

Instance methods always have the first argument as self, can work with object attributes.
Class methods always have the first argument as cls, operate on the class or factory.
Static methods do not have implicit first arguments.

Tricky questions.

Can an instance method be called directly through the class?

Yes, but the object must be explicitly passed:

MyClass.instance_method(obj)

which is rarely appropriate.

Behavior of static methods in inheritance: can they be overridden?

Yes, you can declare a staticmethod in the child class with the same name, and that one will be called when accessed from the descendant.

Why use class methods if static methods with cls as an argument can always be used?

cls is not just the first argument: in classmethod, Python automatically provides the appropriate class for cls even if the call is made from a descendant. This allows creating an alternative hierarchy of constructors without being closely tied to the parent.

Example:

class Base:
    @classmethod
    def make(cls):
        return cls()
class Child(Base):
    pass
Child.make() # will return Child, not Base

Common errors and anti-patterns

Using static methods for operations that depend on the internal state of the object or class.
Incorrect order of arguments (missing self or cls).
Mixing logic of different method types in one class.

Real-life example

Negative case

In a project, regular methods were used for creating alternative instances (e.g., create_from_json). As a result, when inherited, the method always returned an object of the base class, not the descendant.

Pros:

Simple implementation

Cons:

Limited support for inheritance, tight coupling with parent.

Positive case

Classmethod factories were implemented, which return instances of the current class (cls()), even when called from the descendant.