Python Lambda Function

Lambda functions are simple, concise, and anonymous functions. Lambda functions works just like regular functions which are defined with the def keyword. Anywhere a regular function can be used, a lambda function can be used too.

Let's look at a very simple example of a regular function and a lambda function that does the same thing to compare and see their main differences.

The regular function 'add' below, just simply take two parameters and add them together and return the result.

Now, we define a lambda function, by using the keyword lambda, then declare that this lambda function takes two parameter x and y (before the colon), and this lambda function will return the result of these two parameters are added together (after the colon). Then we immediately call to execute this lambda function, by using the parenthesis after it and pass in two numeric parameters 1 and 2.

So, we can easily see that the first difference is the lambda function does not have a name (anonymous) and it can be invoked to execute immediately as the same time it is defined.

In the other hand, a regular function must be defined first and a proper name, before it can be called.

Any things we can do with lambda functions we can always do the same with regular functions, and regular functions are normally have more clarity and better organized.

So, why do we ever need lambda functions at all?

In some cases, we find that lambda functions are very concise and convenient to use.

Instead of going through the hassle of defining a full-fledged function, just to do a very simple, single-expression statement, it is more convenient and some times more intuitive to just define a lambda function inline.

However, if we overuse the lambda functions, it can make the code more ambiguous and not well organized.

A good example of using lambda function that I use all the time is to sort a dictionary based on keys or values that we want. Let look at an example like that below.

We have a list of dictionaries of movies, each dictionary object has key/value pairs of a movie's title, year and revenue.

If we want to sort this list, we have to provide a function to compare any of two movie dictionaries because the dictionary object can't be compared to each other by default. And, even if it has that default comparison, it's most likely not what we want.

For example, we want to sort the movies list by movie titles as below.

Or, we can sort our list by the years to see the movies in the order of released years.

Or, we can sort them by their revenues to see which movie made more money.

There are some other very useful uses of lambda functions, for example I use it a lot to transform data in a column of a Pandas DataFrame.

There are some key takeaways that I'd like to emphasize about lambda function:

• Lambda functions are single-expression function that does not have a name and can be executed immediately without the need of being defined first before use.

• Lambda functions can't use regular Python statements, and always have an implicit return.

• Don't overuse lambda functions because it can cause ambiguity.

Please refer to the full source code of all code above here.

Happy Pythoning!