Duck Typing

 1 programming for data scientists

In an attempt to bridge the gap between the two disciplines of programming and data science I will occasionally talk about programming concepts useful for data scientists, and vice versa.

Today I want to discuss duck typing.

Duck typing is a concept that originated in the Python community. It is a way of checking an object’s type not by testing its type directly, but testing its methods.

The idea is based on something called the duck test. You’ve probably heard it before:

“If it looks like a duck, swims like a duck, and quacks like a duck, then it probably is a duck.”

How does this relate to programming?

Well, Python is a dynamically-typed language. That means that the types of objects (whether they’re integers, strings etc.) is checked at runtime, not compile time.

A variable is allowed to have different types at different points of a program’s execution. This is perfectly valid:

my_variable = 42
# do stuff with my_variable...
# and later...
my_variable = "now it's a string!"

You can’t do this in a statically-typed language. Once you declare a variable as a certain type, it stays that way. Take this example in C#:

int my_variable = 42; // explicitly declare an integer
// do stuff
my_variable = "can I be a string?"; // <- this will produce a compiler error

Actually there are dynamic types in C# but we’ll just gloss over that.

The point is you can be quite liberal with types in Python.

You can take this one step further with duck typing.

Duck Typing in Python

Say you have a function that makes a duck quack, like this:

def make_it_quack(something_duck_like):

We’ve taken an object in and called its quack method. We don’t care what type of object this is, only that it is able to quack. So if we had a “real” duck and an impostor, they’d both work with this method:

class Duck(object):
    def quack(self):
        print("Quack quack")

class Ferret(object):
    # ferrets can't normally quack, but this one's cunning
    def quack(self):
        print("Quack quack")

donald = Duck()
fred = Ferret()


Both of these will produce the output “Quack quack” because all we did was make it quack. If it can do that, then as far as we’re concerned it’s a duck.

Duck Typing in Data Science

This is a concept that can be quite useful in data science.

For example, imagine that you have your own implementation of a machine learning algorithm but want to use a lot of the goodness built in to scikit-learn.

Well, you know how all scikit-learn implementations have a fit and predict function?

You can create your own object and make use of duck typing by “quacking like a scikit-learn duck”.

First, create a class that has fit and predict methods:

class MyFakeClassifier():
    def fit(self, x, y):
        print("Working VERY HARD...")
    def predict(self, x):
        # predict 0 no matter what
        return [0 for item in range(len(x))]

Now you can fit and predict the same way as you could with, say, a Random Forest.

Imagine you already wrote a small function that takes in a machine learning classifier, does a train-test split and gets the accuracy and confusion matrix of the predictions.

from sklearn.datasets import load_iris
from sklearn.model_selection import train_test_split
from sklearn.metrics import accuracy_score, confusion_matrix

# write a function to give us a train-test accuracy score
def get_accuracy(model, X, y):
    X_train, X_test, y_train, y_test = train_test_split(X, y, test_size=0.3, stratify=y), y_train)
    y_pred = model.predict(X_test)
    print("Accuracy: {}\n{}".format(accuracy_score(y_test, y_pred),
                                    confusion_matrix(y_test, y_pred)))

We can use this for a built-in classifier, but also our new estimator:

from sklearn.datasets import load_iris
from sklearn.ensemble import RandomForestClassifier

iris = load_iris()
X =
y =

rf = RandomForestClassifier()
random_model = MyFakeClassifier()

# we can get the accuracy of our Random Forest
get_accuracy(rf, X, y)
# and our new model!
get_accuracy(random_model, X, y)

There you have it. All we had to do was create something that can “fit” and “predict” and Python doesn’t need anything else for it to work.

Note: to use the full range of scikit-learn functions with your own estimator, you should do it properly, but the point is you can do a lot of it by duck typing.

Here’s the associated notebook.

Happy quacking!


Footnote: This is the 14th entry in my 30 day blog challenge.

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