In Python, a function is a separate chunk of code that performs a specific task or set of tasks. To define a function, use the def statement.

For example, here is a definition of a very simple function:

def print_HL7_field_delimiter():
	print "|"

print_HL7_field_delimiter()

The def statement defines a function named print_HL7_field_delimiter that prints an HL7 field delimiter character on the screen. Note that statements inside functions, such as the print statement in this example, must always be indented.

The print_HL7_field_delimiter() statement tells Python to execute the code contained in the print_HL7_field_delimiter function. This is sometimes referred to as a function call or as an invocation of the function.

In Python, a function must be defined before it can be called. The following code generates an error:

print_HL7_field_delimiter()

def print_HL7_field_delimiter():
	print "|"

The print_HL7_field_delimiter() statement generates an error because Python has not yet seen a definition of print_HL7_field_delimiter.

	Function names follow the same rules as variable names: they can be any collection of letters, digits and underscore (_) characters, except that the first character cannot be a digit. You cannot use reserved words (such as if) as function names. You can use a name as both a function name and a variable name, as Python can always figure out which is which. But you are probably better off not doing this, as it may lead to confusion.

Note that functions can call other functions. This enables you to modularize your program: in other words, divide your task into smaller tasks, which can then be divided into smaller tasks, and so on.