Python Lambdas

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Lambda expressions, typically called lambda functions in most Python documentation, are effectively a syntactic shortcut for defining a function within Python code. This is because normal Python functions are already first-class citizens in the language - we can already pass existing named functions as arguments to other functions! So, lambdas in Python are simply shortcuts we can use to create a new anonymous function where needed, but we can always use normal functions to perform the same task.

Python Functions vs. Lambdas

Python lambda functions are effectively the same as Python functions. For example, we can write an addition function in Python in the following way:

def addition(x, y):
    return x + y

The same concept can be expressed as a lambda function, and we can even store it in a variable:

addition_lambda = lambda x, y: x + y

Those two functions are effectively identical - they produce the same result, and can be treated as variables as well as callable functions.

The basic syntax of a lambda function in Python includes the following:

1. The keyword lambda
2. A list of parameters separated by commas, which can be named, positional, keyword, or variable parameters. Basically, any way you can define the parameters for a normal Python function can also be used for a lambda function.
3. A colon after the parameters:
4. A single expression that creates the result of the lambda function. Lambdas may not include multiple expressions, or any statements such as return or pass.

In addition, Python lambda functions are not compatible with type annotations. So, when working with object-oriented Python, we will almost always prefer to write our own functions using the normal syntax, which allows us to perform type checking using Mypy.

Python Lambda Example

Here’s a quick example of using both lambda functions and normal class functions as first-class citizens in Python. This example is adapted from a similar example given in Lambda Expressions from the Oracle Java Tutorials:

class Calculator:
    
    @staticmethod
    def addition(x, y):
        return x + y
    
    def operate_binary(self, a, b, operation):
        return operation(a, b)
    
    @staticmethod
    def main():
        calc = Calculator()
        subtraction = lambda x, y: x - y
        print("40 + 2 = {}".format(calc.operate_binary(40, 2, Calculator.addition)))
        print("20 - 10 = {}".format(calc.operate_binary(20, 10, subtraction)))
        print("7 * 6 = {}".format(calc.operate_binary(7, 6, lambda: x, y: x * y)))

if __name__ == "__main__":
    Calculator.main()

In this code, we are defining two different functions that we’ll use later as arguments:

• addition is a static method within the Calculator class that adds two values together.
• subtraction is a variable in the main function that is storing a lambda function that will subtract two values.

Then, we’ve created a higher-order function operate_binary in the Calculator class, which accepts two integers as parameters a and b, as well as a callable object in the operation parameter. In effect, the operation parameter is meant to be a function, either a traditional Python function or a lambda function.

In our main function, we call calc.operate_binary in two different ways. On the first line, we provide Calculator.addition as the third argument. Notice that we are not including the parentheses at the end of the function name. In that way, we aren’t calling the function Calculator.addition, but we are referencing it as an attribute within the Calculator class. We can do this because functions are first-class citizens in Python, so we can treat them just like any other variable. Inside the calc.operate_binary function, we see that it calls the function stored in the operation variable by putting parentheses after the name, pass in any arguments as needed.

In the second example, we are passing the subtraction variable, which is a lambda function we created earlier, to the calc.operate_binary higher order function. So, it will be stored in operation and executed there.

Finally, we can create an anonymous lambda function directly within the function call to calc.operate_binary. This is why, typically, most lambda functions in Python are thought of as anonymous functions - we don’t give them a name or store them in a variable, we simply create them as needed when we pass them to higher-order functions.

For more information on using lambda functions in Python, check out the references linked below.

References

• How to Use Python Lambda Functions from Real Python
• Python Lambda Functions from GeeksforGeeks
• Python Lambda from w3schools
• Functional Programming HOWTO from Python Documentation