In programming language design, a first-class citizen (also type, object, entity, or value) in a given programming language is an entity which supports all the operations generally available to other entities. These operations typically include being passed as an argument, returned from a function, and assigned to a variable. In computer science, a programming language is said to have first-class functions if it treats functions as first-class citizens.

Functional languages — languages where functions are the primary means of building programs — all have first class functions.

When you say that a language has first-class functions, it means that the language treats functions as values – that you can assign a function into a variable, pass it around etc. Higher-order functions are functions that work on other functions, meaning that they take one or more functions as an argument and can also return a function.

The “higher-order” concept can be applied to functions in general, like functions in the mathematical sense. The “first-class” concept only has to do with functions in programming languages. It’s seldom used when referring to a function, such as “a first-class function”. It’s much more common to say that “a language has/hasn’t first-class function support”.
First-class value can be

• passed as an argument
• returned from a subroutine
• assigned into a variable

Second-class value just can be passed as an argument.
Third-class value even can’t be passed as an argument.

When it comes to functional programming, Python is classified as a “hybrid” language. It supports the functional programming paradigm, but equally supports imperative paradigms (both procedural and object-oriented). Other languages with built-in support for programming in a functional paradigm as well as other paradigms are: F#, C#, Scala, Java 8.

Take Away
Functional programming proved to be booster to success of big data revolution. The main reason is that functional programming facilitates distributed programming.
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