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In software engineering, a class diagram [1] in the Unified Modeling Language (UML) is a type of static structure diagram that describes the structure of a system by showing the system's classes, their attributes, operations (or methods), and the relationships among objects. The class diagram is the main building block of object-oriented modeling.
Python allows the creation of class methods and static methods via the use of the @classmethod and @staticmethod decorators. The first argument to a class method is the class object instead of the self-reference to the instance. A static method has no special first argument. Neither the instance, nor the class object is passed to a static method.
Other implementations are possible: for example, objects in Python use associative key-value containers. [4] Some programming languages such as Eiffel support specification of invariants as part of the definition of the class, and enforce them through the type system.
A class diagram exemplifying the singleton pattern.. In object-oriented programming, the singleton pattern is a software design pattern that restricts the instantiation of a class to a singular instance.
A sample UML class and sequence diagram for the Decorator design pattern. [7] In the above UML class diagram, the abstract Decorator class maintains a reference (component) to the decorated object (Component) and forwards all requests to it (component.operation()). This makes Decorator transparent (invisible) to clients of Component.
For example, in Python, any class can implement an __iter__ method and be used as a collection. [3] Type classes in languages like Haskell, or module signatures in ML and OCaml, are used for many of the things that protocols are used for. [clarification needed] In Rust, interfaces are called traits. [4]
A classifier is an abstract metaclass classification concept that serves as a mechanism to show interfaces, classes, datatypes and components.. A classifier describes a set of instances that have common behavioral and structural features (operations and attributes, respectively).
It should be possible to define a new operation for (some) classes of an object structure without changing the classes. When new operations are needed frequently and the object structure consists of many unrelated classes, it's inflexible to add new subclasses each time a new operation is required because "[..] distributing all these operations across the various node classes leads to a system ...