Functions are useful for gathering similar pieces of code in one place. Consider the following mathematical function:

The Python equivalent is as follows:

def f(x):
    return 2*x + 1

In Figure 1.4 Anatomy of a function the  elements of a function block are explained. 

Figure 1.4: Anatomy of a function

Once the function is defined, it can be called using the following code:

f(2) # 5
f(1) # 3