Published Feb 10, 2023
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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 whether an object 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 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.
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 [Duck(), 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.