Python: Using a Lambda as a Class Method

You can use Python lambdas in different contexts, from simple one-liners to more complex applications. One question that sometimes comes up among developers is whether a Python lambda can be used as a class method.

This article answers this question. We will start with a recap of the basic structure of a lambda function, and then dive into a practical example using a Python class.

If you are not familiar with lambda functions, read the first three sections of the Codefather tutorial about lambda functions. Then come back to this article.

Lambda Functions in Python

Before we see how to use lambda functions as class methods, let’s take a quick moment to explore what lambda functions are and how they work in Python.

Lambda functions (also known as anonymous functions) are a type of function defined with the lambda keyword. Unlike regular functions declared with the def keyword, lambda functions are used for small, one-line tasks where defining a full function might be too verbose.

In Python, a lambda function can take one or more arguments and can only have one expression. Below you can see the syntax of a lambda:

lambda arguments: expression

The expression is evaluated and returned when the lambda function is called. The inline nature of lambda functions makes them a popular choice for short operations.

Now let’s talk about utilizing lambda functions within classes. Typically, methods within a class are defined using the def keyword, as they often involve more complex operations and benefit from the structure of a regular function.

But what if you have a class method that is simple enough and hence you can express it with a single line of code? Is it worth using a lambda function instead of a traditional method?

In the next section, we will explore this idea further with a practical example.

Lambda Function Applied to a Class Method

Let’s examine what it means to replace a standard class method with a lambda function and what syntax you will have to use.

This will help us understand not only how to use lambda functions as class methods but also when they might be a suitable choice for your Python classes.

We will define a class called Character that contains a constructor and the run() method that prints a message:

class Character:
    def __init__(self, name, age, weight):
        self.name = name
        self.age = age
        self.weight = weight

    def run(self):
        print('{} starts running!'.format(self.name))

Create an instance of this class called Spartacus and execute the run() method on it:

Spartacus = Character('Spartacus', 35, 90)
Spartacus.run()

Here is the output you get when executing the method:

Spartacus starts running!

Now, let’s replace the run() method with a lambda function:

run = lambda self: print('{} starts running!'.format(self.name))

An important aspect of the line of code above is that we assign the function object returned by the lambda function to the variable run.

Notice also that:

  • We have removed the def keyword because it applies to a regular method but not to a lambda.
  • The argument of the lambda is the instance of the class (self).
  • The expression of the lambda is the body of the original class method that prints a message.

This is the updated class:

class Character:
    def __init__(self, name, age, weight):
        self.name = name
        self.age = age
        self.weight = weight

    run = lambda self: print('{} starts running!'.format(self.name))

Execute the run() method again on the instance of the Character class. Then confirm that the output message is the same.

Spartacus = Character('Spartacus', 35, 90)
Spartacus.run()

[output]
Spartacus starts running!

The output is correct and this shows that you can use a lambda as a class method.

It’s up to you to choose which one you prefer depending on what makes your code easy to maintain and to understand.

Advantages and Disadvantages of Lambdas as Class Methods

So far, we have introduced the concept of lambda functions and seen their basic application in our class. Now, let’s look at the potential advantages of using lambdas as class methods.

Lambda functions are concise, which means that they can make your code shorter and more Pythonic when you can express the method’s functionality in a single line.

However, a disadvantage is that this comes at a cost as one of the key aspects to keep in mind is the readability and maintainability of your code. While lambdas can reduce the length of the code, they can also make it less readable for developers who are not familiar with functional programming.

Another disadvantage is that lambdas are not suitable when a class method requires the following

  • complex logic
  • multiple statements
  • any kind of branching logic (like if-else statements)
  • annotations
  • docstrings

FAQ

Here are some common questions about using Python lambda functions as class methods.

  1. Q: When should I use a lambda function as a class method instead of a regular method?
    • A: Lambda functions as class methods are best suited for simple operations that don’t require complex logic or multiple statements. Use them when you have a short, one-line method that executes a single action or calculation. If your method requires complex logic, it’s better to use a regular method.
  2. Q: Do I need the def keyword when using a lambda as a class method?
    • A: No, you do not use the def keyword when defining a lambda function, including when using it as a class method. Lambda functions are defined using the lambda keyword, which differentiates them from regular functions.
  3. Q: Are docstrings supported when using a lambda function as a class method?
    • A: No, docstrings are not supported when using a lambda function as a class method. Lambda functions do not support this Python feature.

Conclusion

In this tutorial, you have seen how to write code that uses a Python lambda as a class method.

Now you should have a clearer understanding of when and if to use lambda functions in your class methods. Make sure that this aligns with Python’s philosophy of writing clear and readable code.

Related article: Do you want to know more about lambdas? If you haven’t already, go through the Codefather tutorial about Python lambda functions.

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