Duck typing in computer programming is an application of the duck test— "If it walks like a duck and it quacks like a duck, then it must be a duck"— to determine if an object can be used for a particular purpose. With normal typing, suitability is determined by an object's type. In duck typing, an object's suitability is determined by the presence of certain methods and properties, rather than the type of the object itself.
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 fly(self): print("Duck flying") class Sparrow: def fly(self): print("Sparrow flying") class Whale: def swim(self): print("Whale swimming") for animal in Duck(), Sparrow(), Whale(): animal.fly()
Duck flying Sparrow flying AttributeError: 'Whale' object has no attribute 'fly'
In statically typed languagesEdit
Certain mostly statically typed languages such as C# and Boo have features 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 systemsEdit
Structural type systemsEdit
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 interfacesEdit
Protocols and interfaces may provide some of the benefits of duck typing, yet duck typing is distinct in not having an explicit interface 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 typesEdit
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 runtime need to be implemented, while templates require implementations of all methods that can not be proven unreachable at compile time.
Languages like Python, Java and Objective-C are examples of duck typing because it is possible in them to construct new types in runtime via reflection and inspect these object whether they implement certain methods. On the contrary side stand languages that rely on compile-time metaprogramming techniques (like C++ and its template system) and thus are not fit into the category of duck typing; instead, at some point in compilation pipeline, all placeholder types become substituted with some concrete types specified in a particular instantiation. Even though certain type erasure is possible in them, runtime inspection is limited.
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