''' Purpose: introduce modules
'''

# This is a module! A module / library contains function definitions.
# We will import this module to use it in other programs/ tester program.
   # We call functions from this module using harb.function_name(arguments)
   # Remember that arguments fill in parameters to start up the function!
# We have two functions below, sum(a,b) and negate(x)
# Remember that functions are defined like so:
# def function_name(parameters):
   # ''' Here's what the function does ''' # It's useful to have a docstring but not required.
   # function body

# ALL FUNCTIONS HAVE PARENTHESES.

# Remember that variables and functions are identifiers.
# Unique names you as the coder give to the functions/variables to use them later on!
# sum and negate are identifiers (names of the functions) - we will use these same names to call these functions
# later!

# All functions either return the explicit return value or None by default.
# In sum, we have the explicit return value of total.
# So ALLLLLL functions have a return value (what you say to return or None) !!!
# print() is not the same thing as return! !!!

# Definition uses def function_name(parameters)
# Invocation/call uses module.function_name(arguments)
# Order Matters: Arguments are passed into parameters in the order they're in.
   # sum(2,4) gives a 2 and b 4 and sum(4,2) gives a 4 and b 2.
   # 2 and 4 are arguments              4 and 2 are arguments
def sum( a, b ) :                   # function name is sum(), parameters are a and b
   ''' Returns sum of a and b
   '''

   total = a + b                    # compute value of interest

   return total                     # function hands back the total
   # Once the code executes the return statement, everything after it isn't even
   # looked at.

   # return 88
   # print('Here I am')

# If we don't have an explicit return value (return statement), Python
# returns None! Remember t1 stores the return value of the function call.
# What's the return value of sum with no return statement? It's None.
# I repeat: ALL FUNCTIONS RETURN VALUES (what you say to return or None).

def negate( x ) :                   # function name is negate(), parameter is x
   ''' Returns inverse of x
   '''

   inverse = -x                     # compute value of interest

   return inverse                   # function hands back the inverse