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Wednesday July 17th, 2024

What is a lambda function?

In a nutshell, it is an anonymous, inline "throw away" function that can pass any number of arguments (or no arguments at all) but only returns a single executed expression. Further more, it cannot include statements like return, pass, assert, or raise as these will invoke an exception error!

Let's look at a few examples for better clarity! First, we'll start with a simple example that uses a traditional function:

Example 1a (using a traditional function)

def add_to_number(x):
    return x + 7


Output: 15
In line 4, the function add_to_number is called and passes number 8 as an argument, which within the function is assigned as x, added to 7 and the result is returned and printed.

Now let's rewrite the above example using a lambda function!

Example 1b (using the lambda function)

add_to_number = lambda x: x + 7

Output: 15
In this alternate lambda example, add_to_number is simply assigned as the lambda function which takes the argument of 8 (provided within the print statement on line 2), adds it with 7 and returns the end result.

Here is the anatomy of a lambda function:

Lambda function Syntax:

  • lambda Argument(s) : Expression

  • Argument(s): Value(s) that are passed into the function.
  • Expression: a single operation (expression) performed on said argument(s). It's possible to have no expression whatsoever, but is regarded as useless!

  • The keyword lambda always comes first, followed by the colon which always separates the argument(s) from the expression!

As mentioned previously, a lambda function can take any number of arguments:

Example 2 (multiple arguments)

multiply_some_numbers = lambda a, b, c, d: a * b * c * d

Output: 180
The multiply_some_numbers passes four arguments, and from within the lambda function, assigns those arguments to a,b,c and d respectively, all to end up being multiplied together and result returned!

It is also possible to execute a lambda function by simply surrounding it (and likewise, trailing argument(s)) using a pair of parentheses like so:

Example 3 (parentheses)

added_numbers = (lambda x, y: x + y)(2,4)

Output: 6
It is important to note that the first set of parentheses surrounds the lambda function itself, while the second set surrounds the arguments to be passed in!

It is perfectly acceptable to nest lambda functions within other functions!

Example 4 (lambda function nested within a function)

def myfunc(b):
    return lambda a : a * b

result = myfunc(10)

Output: 40
Here, the lambda function is nested within the myfunc one and 'result' is assigned to myfunc and passes 10 as an argument which is used as argument 'a' with the lambda function. The final print line uses 'result' to pass the second argument (10) which in turn is assigned as the argument 'b' in the lambda function. Both arguments are multiplied together and the result is returned.

Lambda function expressions can also be other functions!

Example 5 (a function as an expression)

(lambda *args : sum(args))(3,5,7) #if printed, the output would be 15
Here, the *args can be considered a placeholder for an unknown number of arguments (which in this case ends up being three arguments - 3,5 and 7 within the last set of parentheses), and the sum() function adds them up and the result is returned.

Additionally, you can use a conditional as an expression!

Example 6 (conditional as expression)

myvar = lambda a,b: a if a > b else b

Output: 32
The variable myvar is assigned to the lambda function and passes two arguments (12 and 32), which in turn is assigned to a and b respectively and then checks in the conditional expression and the result is a if it is greater then b, otherwise b is the result!

When Should I use a lambda function?

  • Small trivial, non complicated tasks requiring a single expression returned.
  • Repetitive tasks that are temporary in nature.
  • In conjunction with the points above, you can make use of a lambda function when you need to capitalize on Python Closures! Here is an additional closure source as well as this one.


As you can see, while lambda functions are limited, they definitely have their place and can prove quite useful whenever you need a simple, single returned expression in one easy discardable function! ∎