a1204773 a1204773 - 12 days ago 4
Python Question

Is this duck-typing in Python?

Here is some Ruby code:

class Duck
def help
puts "Quaaaaaack!"
end
end

class Person
def help
puts "Heeeelp!"
end
end

def InTheForest x
x.help
end

donald = Duck.new
john = Person.new
print "Donald in the forest: "
InTheForest donald
print "John in the forest: "
InTheForest john


And, I translated it to Python:

import sys

class Duck:
def help():
print("Quaaaaaack!")

class Person:
def help():
print("Heeeelp!")

def InTheForest(x):
x.help()

donald = Duck()
john = Person()
sys.stdout.write("Donald in the forest: ")
InTheForest(donald)
sys.stdout.write("John in the forest: ")
InTheForest(john)


The result is the same. Does this mean my Python code is using duck-typing? I couldn't find a duck-typing example, so I thought there may be no duck-typing in Python. There is code in Wikipedia, but I couldn't understand it.

Answer

The code does not show the whole story. Duck typing is about trying something and handling exceptions if they occur. As long it quacks, treat it like a duck, otherwise, treat it differently.

try:
    dog.quack()
except AttributeError:
    dog.woof()

This behavior is explained at the top of the wikipedia Duck_typing article following a description of a non-duck-typed language:

In a duck-typed language, the equivalent function would take an object of any type and call that object's walk and quack methods. If the object does not have the methods that are called then the function signals a run-time error. If the object does have the methods, then they are executed no matter the type of the object, evoking the quotation and hence the name of this form of typing.

For your example:

class Person:
    def help(self):
        print("Heeeelp!")

class Duck:
    def help(self):
        print("Quaaaaaack!")

class SomethingElse:
    pass

def InTheForest(x):
    x.help()

donald = Duck()
john = Person()
who = SomethingElse()

for thing in [donald, john, who]:
    try:
        InTheForest(thing)
    except AttributeError:
        print 'Meeowww!'

output:

Quaaaaaack!
Heeeelp!
Meeowww!
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