It's probably been a while since you read about using functions. Read about it again if you need to.

Have a look at this code:

print("************")
print("Hello World!")
print("************")

print("*************")
print("Enter a word:")
print("*************")

word = input()

if word == 'python':
    print("*******************")
    print("You entered Python!")
    print("*******************")
else:
    print("**************************")
    print("You didn't enter Python :(")
    print("**************************")

Then compare it to this code:

print_box("Hello World!")
print_box("Enter a word:")
word = input()
if word == 'python':
    print_box("You entered Python!")
else:
    print_box("You didn't enter Python :(")

That's nice, but where do we get a print_box function like that?

The pass keyword does nothing.

>>> pass
>>> 

Let's use it to define a function that does nothing.

>>> def do_nothing():
...     pass
... 
>>> do_nothing
<function do_nothing at 0x7f56b74e9598>
>>> 

Seems to be working so far, we have a function. Let's see what happens if we call it.

>>> do_nothing()
>>> 

There we go. It did nothing at all.

Maybe we could just do something in the function instead?

>>> def print_hi():
...     print("Hi!")
... 
>>> print_hi()
Hi!
>>> 

It's working. How about printing a variable in the function?

>>> def print_message():
...     print(message)
... 
>>> message = "Hello World!"
>>> print_message()
Hello World!
>>> 

Again, it works. How about setting a variable in the function?

>>> def get_username():
...     username = input("Username: ")
... 
>>> get_username()
Username: me
>>> username
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
NameError: name 'username' is not defined
>>> 

That was weird! Why didn't that work?

So far we have used nothing but global variables. They are called globals because the same variables are available anywhere in our program, even in functions.

>>> a = 1
>>> b = "hi"
>>> c = "hello"
>>> def print_abc():
...     print(a, b, c)
... 
>>> print_abc()
1 hi hello
>>> 

But there are also local variables. They exist only inside functions, and they are deleted when the function exits.

>>> def thingy():
...     d = "hello again, i'm a local variable"
...     print('inside thingy:', d)
... 
>>> thingy()
inside thingy: hello again, i'm a local variable
>>> d
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
NameError: name 'd' is not defined
>>> 

However, modifying a global variable in-place from a function is easy.

>>> foo = ['global foo']
>>> def bar():
...     foo.append('local foo')
... 
>>> bar()
>>> foo
['global foo', 'local foo']
>>> 

This doesn't work if the value is of an immutable type, like string or integer. Fortunately, Python will tell you if something's wrong.

>>> foo = 1
>>> def bar():
...     foo += 1
... 
>>> bar()
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "<stdin>", line 2, in bar
UnboundLocalError: local variable 'foo' referenced before assignment
>>> 

So far our functions seem to be really isolated from the rest of our code, and it sucks! But they really are not as isolated as you might think they are.

Let's think about what the print function does. It takes an argument and prints it. Maybe a custom function could also take an argument?

>>> def print_twice(message):
...     print(message)
...     print(message)
... 
>>> 

This function can be called in two ways:

• Using a positional argument.

  This is the recommended way for functions that take only one or two arguments. I would do this in my code.

  >>> print_twice("hi")
  hi
  hi
  >>> 

  Positional arguments are great for simple things, but if your function takes many positional arguments it may be hard to tell which argument is which.

• Using a keyword argument:

  >>> print_twice(message="hi")
  hi
  hi
  >>> 

  Keyword arguments are great when your function needs to take many arguments, because each argument has a name and it's easy to see which argument is which.

  Also note that there are no spaces around the = sign. That's because we are not assigning to a variable, we are giving the function a keyword argument and it can do whatever it wants with it.

Personally, I would use this function with a positional argument. It only takes one argument, so I don't need to worry about which argument is which.

Now it's time to solve our box printing problem:

def print_box(message):
    print('*' * len(message))
    print(message)
    print('*' * len(message))

What if we want to print different characters instead of always printing stars?

We could change our print_box function to take two arguments:

def print_box(message, character):
    print(character * len(message))
    print(message)
    print(character * len(message))

Then we could change our existing code to always call print_box with a star as the second argument:

print_box("Hello World", "*")
...

But we don't need to change our existing code. We can make the second argument optional by giving it a default value.

def print_box(message, character='*'):
    print(character * len(message))
    print(message)
    print(character * len(message))

We can print a row of stars using the function without specifying a different character in two ways:

• Using a positional argument.

  print_box("Hello World!")

• Using a keyword argument.

  print_box(message="Hello World!")

Or we can give it a different character in a few different ways if we need to:

• Using two positional arguments.

  print_box("Enter a word:", "?")

• Using two keyword arguments.

  print_box(message="Enter a word:", character="?")
  print_box(character="?", message="Enter a word:")

• Using one positional argument and one keyword argument.

  I would probably do this. If an argument has a default value, I like to use a keyword argument to change it if needed.

  print_box("Enter a word:", character="?")

  However, this doesn't work:

  print_box(character="?", "Enter a word:")

  The problem is that we have a keyword argument before a positional argument. Python doesn't allow this. You don't need to worry about this, because if you accidentally call a function like this you will get an error message.

The built-in input function returns a value. Can our function return a value also?

>>> def times_two(x):
...     return x * 2
... 
>>> times_two(3)
6
>>> times_two("hello")
'hellohello'
>>> 

Yes, it can.

Note that returning from a function ends it immediately.

>>> def return_before_print():
...     return None
...     print("This never gets printed.")
... 
>>> return_before_print()
>>> 

If we don't have any return statements or we have a return statement that doesn't specify what to return, our function will return None.

>>> def return_none_1():
...     pass
... 
>>> def return_none_2():
...     return
... 
>>> print(return_none_1())
None
>>> print(return_none_2())
None
>>> 

There is a lot to learn with functions, and I don't expect you to learn everything at once. However, there's also lots of free Python exercises about defining functions you can do. Do many of them and spend a lot of time with them, so you'll get used to defining functions.

1. Python comes with many built-in functions. Some of the simplest ones are abs, all and any. They can be used like this:

   • abs returns the absolute value of its only argument.

     >>> abs(1)
     1
     >>> abs(-1)
     1
     >>> 

   • any returns True if any of the elements of a list is true, and False otherwise.

     >>> any([True, False, True])
     True
     >>> any([False, False, False])
     False
     >>> 

   • all returns True if all elements of a list are true, and False otherwise.

     >>> all([True, True, True])
     True
     >>> all([True, False, True])
     False
     >>> 

   Define functions my_abs, my_all and my_any that work the same way without using the built-in functions. Then run the program with IDLE, or with py -i file.py on Windows or python3 -i file.py on other operating systems. Try the above examples with your functions.

2. The box printing function doesn't really print a box, it prints a message between two lines.

    ************
    Hello World!
    ************
   

   Modify it to print an actual box:

    ****************
    * Hello World! *
    ****************
   

3. Back in Lists, you needed to skip parts of the tutorial and all the exercises because you didn't know how to define functions. Read those parts now, and do the exercises.

4. Use a search engine (e.g. Google) to find more exercises about defining functions.

Answers for the first and second exercise are here.