A function is always defined with parentheses. The parentheses are used to pass any requisite arguments to a function. Arguments are parameters required to execute a function. In the earlier example, there are no arguments passed to the function.

Let's review an example where we pass an argument to a function:

def add_function(a, b): 
  c = a + b 
  print("The sum of a and b is ", c)

In this example, a and b are arguments to the function. The function adds a and b and prints the sum on the screen. When the function add_function is called by passing the arguments 3 and 2 as add_function(3,2) where a is 3 and b is 2, respectively.

Hence, the arguments a and b are required to execute function, or calling the function without the arguments would result in an error. Errors related to missing arguments could be avoided by setting default values to the arguments:

def add_function(a=0, b=0): 
  c = a + b 
  print("The sum of a and b is ", c)

The preceding function expects two arguments. If we pass only one argument to this function, the other defaults to zero. For example, add_function(a=3), b defaults to 0, or add_function(b=2), a defaults to 0. When an argument is not furnished while calling a function, it defaults to zero (declared in the function).

Similarly, the print function prints any variable passed as an argument. If the print function is called without any arguments, a blank line is printed.