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Duck typing in computer programming is an application of the
duck test The duck test is a form of abductive reasoning, usually expressed as "If it looks like a duck, swims like a duck, and quacks like a duck, then it probably is a duck." The test implies that a person can identify an unknown subject by observing t ...
—"If it walks like a duck and it quacks like a duck, then it must be a duck"—to determine whether an
object Object may refer to: General meanings * Object (philosophy), a thing, being, or concept ** Object (abstract), an object which does not exist at any particular time or place ** Physical object, an identifiable collection of matter * Goal, an ...
can be used for a particular purpose. With nominative typing, an object is ''of a given type'' if it is declared to be (or if a type's association with the object is inferred through mechanisms such as object inheritance). In duck typing, an object is ''of a given type'' if it has all
methods Method ( grc, μέθοδος, methodos) literally means a pursuit of knowledge, investigation, mode of prosecuting such inquiry, or system. In recent centuries it more often means a prescribed process for completing a task. It may refer to: *Scien ...
and properties required by that type. Duck typing can be viewed as a usage-based structural equivalence between a given object and the requirements of a type. See structural typing for a further explanation of structural type equivalence.


Example

This is a simple example in Python 3 that demonstrates how any object may be used in any context, up until it is used in a way that it does not support. class Duck: def swim(self): print("Duck swimming") def fly(self): print("Duck flying") class Whale: def swim(self): print("Whale swimming") for animal in uck(), Whale() animal.swim() animal.fly() Output: Duck swimming Duck flying Whale swimming AttributeError: 'Whale' object has no attribute 'fly' So, if we assume everything that can swim is a duck because ducks can swim, we will consider a whale to be a duck, but, if we also assume it has to be capable of flying, the whale won’t be considered to be a duck.


In statically typed languages

In some statically typed languages such as Boo and D, class type checking can be specified to occur at run time rather than compile time.


Comparison with other type systems


Structural type systems

Duck typing is similar to, but distinct from, structural typing. Structural typing is a static typing system that determines type compatibility and equivalence by a type's structure, whereas duck typing is
dynamic Dynamics (from Greek δυναμικός ''dynamikos'' "powerful", from δύναμις ''dynamis'' "power") or dynamic may refer to: Physics and engineering * Dynamics (mechanics) ** Aerodynamics, the study of the motion of air ** Analytical dyn ...
and determines type compatibility by only that part of a type's structure that is accessed during run time. The
TypeScript TypeScript is a free and open source programming language developed and maintained by Microsoft. It is a strict syntactical superset of JavaScript and adds optional static typing to the language. It is designed for the development of large app ...
, Elm, and Python languages support structural typing to varying degrees.


Protocols and interfaces

Protocols and interfaces provide a way to declare, explicitly, that some methods, operators, or behaviors need to be defined (e.g. must have a ''quack()'' method). If a third-party library implements a class that cannot be modified, a client cannot use an instance of it with an interface unknown to that library even if the class does, in fact, satisfy the interface requirements. A common solution to this problem is the
Adapter pattern In software engineering, the adapter pattern is a software design pattern (also known as wrapper, an alternative naming shared with the decorator pattern) that allows the interface of an existing class to be used as another interface. It is often ...
. In contrast, under duck typing, the object would be accepted directly, without the need for an adaptor.


Templates or generic types

Template Template may refer to: Tools * Die (manufacturing), used to cut or shape material * Mold, in a molding process * Stencil, a pattern or overlay used in graphic arts (drawing, painting, etc.) and sewing to replicate letters, shapes or designs ...
, or
generic Generic or generics may refer to: In business * Generic term, a common name used for a range or class of similar things not protected by trademark * Generic brand, a brand for a product that does not have an associated brand or trademark, other ...
functions or methods apply the duck test in a static typing context; this brings all the advantages and disadvantages of static versus dynamic type checking in general. Duck typing can also be more flexible in that only the methods ''actually called at runtime'' need to be implemented, while templates require implementations of all methods that ''can not be proven unreachable at compile time''. In languages like Java, Scala, and Objective-C,
reflection Reflection or reflexion may refer to: Science and technology * Reflection (physics), a common wave phenomenon ** Specular reflection, reflection from a smooth surface *** Mirror image, a reflection in a mirror or in water ** Signal reflection, in ...
can be used to inspect whether objects implement methods or even add necessary methods at runtime. For example,
Java Java (; id, Jawa, ; jv, ꦗꦮ; su, ) is one of the Greater Sunda Islands in Indonesia. It is bordered by the Indian Ocean to the south and the Java Sea to the north. With a population of 151.6 million people, Java is the world's mo ...
'
MethodHandle API
can be used in this manner.


See also

* Extension method * UFCS * Loose coupling * Monkey patch * Dynamic programming language


References

{{DEFAULTSORT:Duck Typing Articles with example pseudocode Object-oriented programming Type theory Articles with example Python (programming language) code