Python functions

Functions organize a bunch of statements into chunks that we can edit easily and re-use in different places:

def add_stuff(input1, input2, input3):
  sum_inputs = input1 + input2 + input3
  return sum_inputs

Unpacking the above:

• def = keyword that declares the function
• add_stuff = the name of the function
  • one word beginning with a letter but can contain digits and underscores
• (input1, input2, input3) = the parameters of the function
• : = starts the function block
• return = gives back a value to whichever piece of code used the function

We can then use (call) this function like such:

x = add_stuff(3, 2, 5)
print(x)

y = add_stuff(1, 2, 2) + add_stuff(-1, 2, 3)
print(y)

z = add_stuff("a", "b", "c") 
print(z) 

# Output
# 10
# 9 (i.e. 5 + 4)
# abc (i.e. strings glued together)

Note that the arguments refer to the specific values that we insert when calling the function, while the parameters refer to the generic template of those values!

Recursive functions

A recursive function calls itself from within, a classical example being the calculation of a factorial (e.g. 4 factorial = 4! = 4 x 3 x 2 x 1):

def factorial(num):
   if num == 1:
       return 1
   else:
       return num * factorial(num-1)

It has the general idea of a looping structure in that it:

• removes repetition
  • i.e. the else statement recursively calling the function
• contains a "control statement" to break out of it
  • i.e. the if statement above

Recursion in programming and web development primarily happens with nested structures such as objects and arrays (lists)!