What is duck typing

Question

I came across the term duck typing while reading random topics on software online and did not completely understand it   What is    duck typing

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I see a lot of answers that repeat the old idiom:

If it looks like a duck and quacks like a duck, it's a duck

and then dive into an explanation of what you can do with duck typing, or an example which seems to obfuscate the concept further.

I don't find that much help.

This is the best attempt at a plain english answer about duck typing that I have found:

Duck Typing means that an object is defined by what it can do, not by what it is.

This means that we are less concerned with the class/type of an object and more concerned with what methods can be called on it and what operations can be performed on it. We don't care about it's type, we care about what it can do.

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Duck typing       If it talks and walks like a duck  then it is a duck   This is typically called abduction  abductive reasoning or also called retroduction  a clearer definition I think     from C  conclusion  what we see  and R  rule  what we know   we accept decide assume P  Premise  property  in other words a given fact      the very basis of medical diagnosis  with ducks  C   walks  talks  R   like a duck  P   it s a duck   Back to programming    object o has method property mp1 and interface type T requires defines mp1 object o has method property mp2  and interface type T requires defines mp2       So  more than simply accepting mp1    on any object as long has it meets some definition of mp1     compiler runtime should also be okay with the assertion o is of type T  And well  is it the case with examples above  Is Duck typing is essentially no typing at all  Or should we call it implicit typing

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Simple Explanation  without code   Discussion of the semantics of the question is fairly nuanced  and very academic   but here s the general idea   Duck Typing       If it walks like a duck and quacks like a duck then it is a duck      - YES  but what does that mean    This is best illustrated by example   Examples of Duck Typing functionality   Imagine I have a magic wand  It has special powers  If I wave the wand and say  Drive   to a car  well then  it drives   Does it work on other things  Not sure  so I try it on a truck  Wow - it drives too  I then try it on planes  trains and 1 Woods  they are a type of golf club which people use to  drive  a golf ball   They all drive    But would it work on say  a teacup  Error  KAAAA-BOOOOOOM  that didn t work out so good        Teacups can t drive   duh    This is basically the concept of duck typing  It s a try-before-you-buy  system  If it works  all is well  But if it fails  like a grenade still in your hand  it s gonna blow up in your face   In other words  we are interested in what the object can do  rather than with what the object is   Example  statically typed languages  If we were concerned with what the object actually was  then our magic trick will work only on pre-set  authorised types - in this case cars  but will fail on other objects which can drive  trucks  mopeds  tuk-tuks etc  It won t work on trucks because our magic wand is expecting it to only work on cars    In other words  in this scenario  the magic wand looks very closely at what the object is  is it a car   rather than what the object can do  e g  whether cars  trucks etc  can drive     The only way you can get a truck to drive is if you can somehow get the magic wand to expect both trucks and cars  perhaps by  implementing a common interface    If you don t know what that means then just ignore it for the moment   Summary  Key take-out  What s important in duck typing is what the object can actually do  rather than what the object is

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It is a term used in dynamic languages that do not have strong typing   The idea is that you don t need a type in order to invoke an existing method on an object - if a method is defined on it  you can invoke it   The name comes from the phrase  If it looks like a duck and quacks like a duck  it s a duck    Wikipedia has much more information

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I know I am not giving generalized answer  In Ruby  we don   t declare the types of variables or methods    everything is just some kind of object  So Rule is  Classes Aren   t Types   In Ruby  the class is never  OK  almost never  the type  Instead  the type of an object is defined more by what that object can do  In Ruby  we call this duck typing  If an object walks like a duck and talks like a duck  then the interpreter is happy to treat it as if it were a duck   For example  you may be writing a routine to add song information to a string  If you come from a C  or Java background  you may be tempted to write this   def append song result  song        test we re given the right parameters      unless result kind of  String          fail TypeError new  String expected   end     unless song kind of  Song          fail TypeError new  Song expected   end  result  lt  lt  song title  lt  lt        lt  lt  song artist  lt  lt      end result       append song result  song      gt   I Got Rhythm  Gene Kelly     Embrace Ruby   s duck typing  and you   d write something far simpler   def append song result  song      result  lt  lt  song title  lt  lt        lt  lt  song artist  lt  lt      end  result      append song result  song      gt   I Got Rhythm  Gene Kelly     You don   t need to check the type of the arguments  If they support  lt  lt   in the case of result  or title and artist  in the case of song   everything will just work  If they don   t  your method will throw an exception anyway  just as it would have done if you   d checked the types   But without the check  your method is suddenly a lot more flexible  You could pass it an array  a string  a file  or any other object that appends using  lt  lt   and it would just work

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Duck Typing is not Type Hinting   Basically in order to use  duck typing  you will not target a specific type but rather a wider range of subtypes  not talking about inheritance  when I mean subtypes I mean  things  that fit within the same profiles  by using a common interface   You can imagine a system that stores information  In order to write read information you need some sort of storage and information   Types of storage may be  file  database  session etc   The interface will let you know the available options  methods  regardless of the storage type  meaning that at this point nothing is implemented  In another words the Interface doesn t know nothing about how to store information   Every storage system must know the existence of the interface by implementing it s very same methods   interface StorageInterface      public function write string  key  array  value   bool     public function read string  key   array      class File implements StorageInterface       public function read string  key   array             reading from a file            public function write string  key  array  value   bool              writing in a file implementation           class Session implements StorageInterface       public function read string  key   array             reading from a session            public function write string  key  array  value   bool              writing in a session implementation           class Storage implements StorageInterface       private   storage   null       function   construct StorageInterface  storage             this- gt  storage    storage             public function read string  key   array           return  this- gt  storage- gt read  key              public function write string  key  array  value   bool           return   this- gt  storage- gt write  key   value     true   false            So now  every time you need to write read information    file   new Storage new File      file- gt write  filename     information      echo  file- gt read  filename      session   new Storage new Session      session- gt write  filename     information      echo  session- gt read  filename      In this example you end up using Duck Typing in Storage constructor   function   construct StorageInterface  storage        Hope it helped

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Consider you are designing a simple function  which gets an object of type Bird and calls its walk   method  There are two approaches you can think of   This is my function  and I must be sure that it only accepts the Bird  or the code will not compile  If anyone wants to use my function  they must be aware that I only accept Birds My function gets any objects and I just call the object s walk   method  So  if the object can walk   it is correct  if it can t my function will fail  So here it is not important the object is a Bird or anything else  it is important that it can walk     This is duck typing   It must be considered that the duck typing may be useful in some cases  for example Python uses duck typing a lot   Useful reading  There are good examples of duck typing for Java  Python  JavaScript etc at  https   en wikipedia org wiki Duck typing Here is also a good answer which describes the advantages of dynamic typing and also its disadvantages  What is the supposed productivity gain of dynamic typing

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The term Duck Typing is a lie  You see the idiom    If it walks like a duck and quacks like a duck then it is a duck  quot  that is being repeated here time after time  But that is not what duck typing  or what we commonly refer to as duck typing  is about  All that the Duck Typing we are discussing is about  is trying to force a command on something  Seeing whether something quacks or not  regardless of what it says it is  But there is no deduction about whether the object then is a Duck or not  For true duck typing  see type classes  Now that follows the idiom    If it walks like a duck and quacks like a duck then it is a duck  quot   With type classes  if a type implements all the methods that are defined by a type class  it can be considered a member of that type class  without having to inherit the type class   So  if there is a type class Duck which defines certain methods  quack and walk-like-duck   anything that implements those same methods can be considered a Duck  without needing to inherit Duck

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Wikipedia has a fairly detailed explanation   http   en wikipedia org wiki Duck typing     duck typing is a style of dynamic   typing in which an object s current   set of methods and properties   determines the valid semantics  rather   than its inheritance from a particular   class or implementation of a specific   interface    The important note is likely that with duck typing a developer is concerned more with the parts of the object that are consumed rather than what the actual underlying type is

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In duck typing  an object s suitability  to be used in a function  for example  is determined based on whether certain methods and or properties are implemented rather than based on the type of that object  For example  in Python  the len function can be used with any object implementing the   len   method  It doesn t care if that object is of a certain type say string  list  dictionary or MyAwesomeClass  as far as these objects implement a   len   method  len will work with them  class MyAwesomeClass      def   init   self  str           self str   str          def   len   self           return len self str   class MyNotSoAwesomeClass      def   init   self  str           self str   str  a   MyAwesomeClass  quot hey quot   print len a      Prints 3  b   MyNotSoAwesomeClass  quot hey quot   print len b      Raises a type error  object of type  quot MyNotSoAwesomeClass quot  has no len    In other words  MyAwesomeClass looks like a duck and quacks like a duck and therefore is a duck  while MyNotSoAwesomeClass doesn t look like a duck and doesn t quack  therefore is not a duck

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Looking at the language itself may help  it often helps me  I m not a native English speaker    In duck typing   1  the word typing does not mean typing on a keyboard  as was the persistent image in my mind   it means determining  what type of a thing is that thing    2  the word duck expresses how is that determining done  it s kind of a  loose  determining  as in   if it walks like a duck     then it s a duck   It s  loose  because the thing may be a duck or may not  but whether it actually is a duck doesn t matter  what matters is that I can do with it what I can do with ducks and expect behaviors exhibited by ducks  I can feed it bread crumbs and the thing may go towards me or charge at me or back off     but it will not devour me like a grizzly would

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I think it s confused to mix up dynamic typing  static typing and duck typing  Duck typing is an independent concept and even static typed language like Go  could have a type checking system which implements duck typing  If a type system will check the methods of a  declared  object but not the type  it could be called a duck typing language

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Duck typing means that an operation does not formally specify the requirements that its operands have to meet  but just tries it out with what is given   Unlike what others have said  this does not necessarily relate to dynamic languages or inheritance issues   Example task  Call some method Quack on an object   Without using duck-typing  a function f doing this task has to specify in advance that its argument has to support some method Quack  A common way is the use of interfaces  interface IQuack        void Quack       void f IQuack x         x Quack         Calling f 42  fails  but f donald  works as long as donald is an instance of a IQuack-subtype   Another approach is structural typing - but again  the method Quack   is formally specified any anything that cannot prove it quacks in advance will cause a compiler failure   def f x     def Quack     Unit      x Quack      We could even write  f    Quackable a   gt  a - gt  IO    f   quack   in Haskell  where the Quackable typeclass ensures the existence of our method    So how does duck typing change this    Well  as I said  a duck typing system does not specify requirements but just tries if anything works   Thus  a dynamic type system as Python s always uses duck typing   def f x       x Quack     If f gets an x supporting a Quack    everything is fine  if not  it will crash at runtime   But duck typing doesn t imply dynamic typing at all - in fact  there is a very popular but completely static duck typing approach that doesn t give any requirements too   template  lt typename T gt  void f T x    x Quack         The function doesn t tell in any way that it wants some x that can Quack  so instead it just tries at compile time and if everything works  it s fine

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I try to understand the famous sentence in my way   Python dose not care an object is a real duck or not  All it cares is whether the object  first  quack   second  like a duck     There is a good website  http   www voidspace org uk python articles duck typing shtml id14  The author pointed that duck typing let you create your own classes that have their own internal data structure - but are accessed using normal Python syntax

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Tree Traversal with duck typing technique  def traverse t       try          t label       except AttributeError          print t  end          else            Now we know that t node is defined         print      t label    end              for child in t              traverse child          print      end

[programming-languages] What is duck typing?

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