valid code, part three

main.py

# Today I'll tell you about FUNCTIONS. 
# Often, we need to reuse code because we do similar operations and don't wanna repeat the code every single time 
# because that means larger code (that's bad) and awkward and hard to understand code (for humans)
# You call (fancy word for execute) a function using function_name(paremeter1, paremeter2, paremeter3, ...). 
# Functions can have anywhere from zero to virtually infinite paremeters

def countdown(start_number): 
    if start_number > 0:  # only do a countdown if the start number is greater than zero
        print(start_number)  # print the starting number
        countdown(start_number-1)  # what's this? It's a functions that calls itself!
                                   # How does it work, you ask? 
# Well think about it!
# Say we start with some code:
countdown(3)
print("BLAST OFF!!")
# Then it checks if 3 > 0
# Since that's true, we print "3".
# Now we call countdown(2)
# The function again checks if 2 > 0
# Since that's true, we print "2"
# Now we call countdown(1)
# The function again checks if 1 > 0
# Since that's true, we print "1"
# Now we call countdown(0)
# The function again checks if 0 > 0
# It isn't. There isn't any code to execute after "if", so we exit into the next level.
# We go up one level to the code that called the function
# There is no code to execute after that function call, so we exit into the next level.
# and so on until the top most level (`countdown(3)`)
# Then we go up one level to the code that called the function. We step onto the next level.
# The next instruction tells us to write "BLAST OFF!!" to the console.

# the output would be:

# 3
# 2
# 1
# BLAST OFF!

# Note that there isn't a one-second pause between each printed number because we didn't tell the computer to do that
# Remember, it ignores names like "countdown"! The computer doesn't know english. (stupid idiot computer... ^jk)
# If we wanted to do that, we'd need to write `import time` at the top of the file and then `time.sleep(1)` between
# the `print()` and the `countdown(n-1)`

# and in this paragraph, we = computer. Sorry if that was confusing.

# we could've done:
print(3)
print(2)
print(1)
print("BLAST OFF!")
# but that's too manual and boring.