In computer programming, duck typing is an application of the duck test in type safety. It requires that type checking be deferred to runtime, and is implemented by means of dynamic typing or reflection.
Duck typing is concerned with establishing the suitability of an object for some purpose, using the principle, "If it walks like a duck and it quacks like a duck, then it must be a duck." With normal typing, suitability is assumed to be determined by an object's type only. In duck typing, an object's suitability is determined by the presence of certain methods and properties (with appropriate meaning), rather than the actual type of the object.
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 Parrot: def fly(self): print("Parrot flying") class Airplane: def fly(self): print("Airplane flying") class Whale: def swim(self): print("Whale swimming") def lift_off(entity): entity.fly() parrot = Parrot() airplane = Airplane() whale = Whale() lift_off(parrot) # prints `Parrot flying` lift_off(airplane) # prints `Airplane flying` lift_off(whale) # Throws the error `'Whale' object has no attribute 'fly'`
In statically typed languages
Certain usually statically typed languages such as Boo and the version 4 release of C# have extra type annotations that instruct the compiler to arrange for type checking of classes to occur at run-time rather than compile time, and include run-time type checking code in the compiled output.
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 and determines type compatibility by only that part of a type's structure that is accessed during run time.
Protocols and Interfaces
Protocols and interfaces can provide some of the benefits of duck typing, but duck typing is distinct in that no explicit interface is defined. For example, 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.) Duck typing would allow this. Again, all of an interface must be satisfied for compatibility.
Templates or generic types
Template, or generic 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 run time need to be implemented, while templates require implementation of all methods that cannot be proven unreachable at compile time.
Criticism of the term itself
Use of the term "duck typing" has been considered superfluous in light of the fact that other terms, such as dynamic binding, express the concept more clearly. To proponents of static type checking, duck typing suggests the absence of typing, making its incorporation of the term typing appear incoherent.
- Boo: Duck Typing Archived October 6, 2008, at the Wayback Machine.
- "Anders Hejlsberg Introduces C# 4.0 at PDC 2008". Retrieved 30 January 2017.
- Czaplicki, Evan. "Core Language · An Introduction to Elm". Retrieved 30 January 2017.
- Lippert, Eric (2 Jan 2014). "What is "duck typing"?". Fabulous adventures in coding. Retrieved 25 May 2016.
... the whole idea of duck typing is fundamentally incoherent ...'