Now that we've clarified how to use functions, how can we create one of our own? Let's write our own function to add two numbers together in order to see how this works.

When defining a function, you have to start with the keyword def, and then give it a name. Names for functions should follow all the same rules as names for variables.

After the function name, we have a pair of parentheses with any parameters inside them. Here, instead of giving specific values, we use new variable names. The order we place the variables inside the parentheses here determines the order that we need to follow when using the finished function later on.

Just like with if statements, we end this line with a colon and indent anything that goes in the body of the function underneath.

In our function, we want to add the two numbers together, so we can add a line like result = a + b. If we call the function now, though, and try to print the value of result, it won't work:

def sum(a, b):
  result = a + b

sum(5, 9)
print(result)

# Traceback (most recent call last):
#   File "<stdin>", line 1, in <module>
# NameError: name 'result' is not defined

In order to access the value of that variable, we first have to return it.

return is another Python keyword with a special meaning. Its job is to take a value from inside the function and bring it outside. So in our example, if we return the value of result, we can then use that returned value to create and print the variable total:

def sum(a, b):
  result = a + b
  return result

total = sum(5, 9)
print(total)
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