Duck typing

<h2 id="examples">Examples</h2>
<p>This simple example in <a href="/facts/Python_(programming_language)/YbuGqofa">Python</a> 3 demonstrates how any object may be used in any context until it is used in a way that it does not support.
</p>
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 [Duck(), Whale()]:
    animal.swim()
    animal.fly()

<p>Output:
</p>
Duck swimming
Duck flying
Whale swimming
AttributeError: 'Whale' object has no attribute 'fly'

<p>If it can be assumed that anything that can swim is a duck because ducks can swim, a whale could be considered a duck; however, if it is also assumed that a duck must be capable of flying, the whale will not be considered a duck.
</p>
<h2 id="in-statically-typed-languages">In statically typed languages</h2>
<p>In some <a href="/facts/Type_system/Tz0bAMS9">statically typed</a> languages such as <a href="/facts/Boo_(programming_language)/CB2lRUwl">Boo</a><a class="footnote-ref" id="fnref:3" href="#fn:3"><sup>3</sup></a> and <a href="/facts/D_(programming_language)/4qIzRjkF">D</a>,<a class="footnote-ref" id="fnref:4" href="#fn:4"><sup>4</sup></a><a class="footnote-ref" id="fnref:5" href="#fn:5"><sup>5</sup></a> class type checking can be specified to occur at runtime rather than at compile time.
</p>
<h2 id="comparison-with-other-type-systems">Comparison with other type systems</h2>
<h3>Structural type systems</h3>
<p>Duck typing is similar to, but distinct from, <a href="/facts/Structural_type_system/1JH0azFU">structural typing</a>. Structural typing is a <a href="/facts/Type_system/Tz0bAMS9">static typing</a> system that determines type compatibility and equivalence by a type's structure, whereas duck typing is <a href="/facts/Type_system/Tz0bAMS9">dynamic</a> and determines type compatibility by only that part of a type's structure that is accessed during <a href="/facts/Runtime_(program_lifecycle_phase)/lCxKsrzB">runtime</a>.
</p><p>The <a href="/facts/TypeScript/W9J27V1N">TypeScript</a>,<a class="footnote-ref" id="fnref:6" href="#fn:6"><sup>6</sup></a> <a href="/facts/Elm_(programming_language)/TnQVYGn6">Elm</a><a class="footnote-ref" id="fnref:7" href="#fn:7"><sup>7</sup></a> and <a href="/facts/Python_(programming_language)/YbuGqofa">Python</a><a class="footnote-ref" id="fnref:8" href="#fn:8"><sup>8</sup></a> languages support structural typing to varying degrees.
</p>
<h3>Protocols and interfaces</h3>
<p><a href="/facts/Interface_(object-oriented_programming)/jDMpXoLN">Protocols and interfaces</a> provide a way to explicitly declare that some methods, operators or behaviors must be defined. 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 satisfies the interface requirements. A common solution to this problem is the <a href="/facts/Adapter_pattern/jbLgy7ct">adapter pattern</a>. In contrast, with duck typing, the object would be accepted directly without the need for an adapter.
</p>
<h3>Templates or generic types</h3>
<p><a href="/facts/Template_metaprogramming/yybwJ7AN">Template</a> (also called <a href="/facts/Generic_programming/kqtqJk7p">generic</a>) functions or methods apply the duck test in a <a href="/facts/Type_system/Tz0bAMS9">static typing</a> context; this brings all of the advantages and disadvantages of <a href="/facts/Type_system/Tz0bAMS9">static versus dynamic type checking</a>. Duck typing can also be more flexible in that only the methods actually called at runtime must be implemented, while templates require implementations of all methods that cannot be proven <a href="/facts/Unreachable_code/BNas1LeK">unreachable</a> at compile time.
</p><p>In languages such as Java, Scala and Objective-C, <a href="/facts/Reflection_(computer_programming)/Nj8stFfp">reflection</a> may be employed to inspect whether objects implement methods or add necessary methods at runtime. For example, <a href="/facts/Java_(programming_language)/9ScgFyAL">Java</a>'s MethodHandle API can be used in this manner.<a class="footnote-ref" id="fnref:9" href="#fn:9"><sup>9</sup></a>
</p>
<h2 id="see-also">See also</h2>
<ul><li><a href="/facts/Ad_hoc_polymorphism/C15g3Bvz">Ad hoc polymorphism</a></li>
<li><a href="/facts/Dynamic_dispatch/211NC5eu">Dynamic dispatch</a></li>
<li><a href="/facts/Dynamic_programming_language/j8cQm8RF">Dynamic programming language</a></li>
<li><a href="/facts/Extension_method/fkvzp3Nx">Extension method</a></li>
<li><a href="/facts/Loose_coupling/uXRb4qBJ">Loose coupling</a></li>
<li><a href="/facts/Monkey_patch/DEIgDszg">Monkey patch</a></li>
<li><a href="/facts/Operator_overloading/hRyHs4Qf">Operator overloading</a></li></ul>

<h2 id="references">References</h2>

<ol>
<li id="fn:1"><p>"Glossary — Python 3.7.1 documentation". docs.python.org. Retrieved 2018-11-08. <a href="https://docs.python.org/3/glossary.html#term-duck-typing" target="_blank">https://docs.python.org/3/glossary.html#term-duck-typing</a> <a href="#fnref:1" class="footnote-back-ref">↩</a></p></li>
<li id="fn:2"><p>"Python Duck Typing - Example". Techie Hours. 2020-06-28. Archived from the original on 2022-03-31. Retrieved 2020-07-26. <a href="https://web.archive.org/web/20220331075920/https://techiehours.com/python/python-duck-typing/" target="_blank">https://web.archive.org/web/20220331075920/https://techiehours.com/python/python-duck-typing/</a> <a href="#fnref:2" class="footnote-back-ref">↩</a></p></li>
<li id="fn:3"><p>Boo: Duck TypingArchived October 6, 2008, at the Wayback Machine <a href="http://boo.codehaus.org/Duck+Typing" target="_blank">http://boo.codehaus.org/Duck+Typing</a> <a href="#fnref:3" class="footnote-back-ref">↩</a></p></li>
<li id="fn:4"><p>"Dynamic classes and duck typing". <a href="https://forum.dlang.org/post/hf1e1h$81s$1@digitalmars.com" target="_blank">https://forum.dlang.org/post/hf1e1h$81s$1@digitalmars.com</a> <a href="#fnref:4" class="footnote-back-ref">↩</a></p></li>
<li id="fn:5"><p>"Metaprogramming - duck typing in D". <a href="https://stackoverflow.com/questions/16578676/duck-typing-in-d" target="_blank">https://stackoverflow.com/questions/16578676/duck-typing-in-d</a> <a href="#fnref:5" class="footnote-back-ref">↩</a></p></li>
<li id="fn:6"><p>"SE Radio Episode 384: Boris Cherny on TypeScript". se-radio.net. Retrieved 2019-10-25. <a href="https://www.se-radio.net/2019/10/episode-384-boris-cherny-on-typescript" target="_blank">https://www.se-radio.net/2019/10/episode-384-boris-cherny-on-typescript</a> <a href="#fnref:6" class="footnote-back-ref">↩</a></p></li>
<li id="fn:7"><p>Czaplicki, Evan. "Core Language · An Introduction to Elm". Retrieved 30 January 2017. <a href="http://guide.elm-lang.org/core_language.html" target="_blank">http://guide.elm-lang.org/core_language.html</a> <a href="#fnref:7" class="footnote-back-ref">↩</a></p></li>
<li id="fn:8"><p>"PEP 544 – Protocols: Structural subtyping (static duck typing)". <a href="https://peps.python.org/pep-0544/" target="_blank">https://peps.python.org/pep-0544/</a> <a href="#fnref:8" class="footnote-back-ref">↩</a></p></li>
<li id="fn:9"><p>"StackOverflow: Implement duck typing using java MethodHandles". Retrieved 13 June 2020. <a href="https://stackoverflow.com/questions/62321273/implement-duck-typing-using-java-methodhandles" target="_blank">https://stackoverflow.com/questions/62321273/implement-duck-typing-using-java-methodhandles</a> <a href="#fnref:9" class="footnote-back-ref">↩</a></p></li>
</ol>

Duck typing open-in-new

Duck typing