Functions are the structured or procedural programming way of organizing the logic in your programs. Large blocks of code can be neatly segregated into manageable chunks, and space is saved by putting oft-repeated code in functions as opposed to multiple copies everywhere — this also helps with consistency because changing the single copy means you do not have to hunt for and make changes to multiple copies of duplicated code. The basics of functions in Python are not much different from those of other languages.

Functions are often compared to procedures. Both are entities that can be invoked, but the traditional function or “black box,” perhaps taking some or no input parameters, performs some amount of processing, and concludes by sending back a return value to the caller. Some functions are Boolean in nature, returning a “yes” or “no” answer, or, more appropriately, a non-zero or zero value, respectively. Procedures are simply special cases, functions that do not return a value. Python procedures are implied functions because the interpreter implicitly returns a default value of None.

A) def Statement

Functions are created using the def statement, with a syntax like the following:

The header line consists of the def keyword, the function name, and a set of arguments (if any). The remainder of the def clause consists of an optional but highly recommended documentation string and the required function body suite. 

Some programming languages differentiate between function declarations and function definitions. A function declaration consists of providing the parser with the function name, and the names (and traditionally the types) of its arguments, without necessarily giving any lines of code for the function, which is usually referred to as the function definition.

In languages where there is a distinction, it is usually because the function definition may belong in a physically different location in the code from the function declaration. Python does not make a distinction between the two, as a function clause is made up of a declarative header line immediately followed by its defining suite.