Python Programming: Advanced Function Arguments

Lesson Objectives

After this lesson, you will be able to…

Review all topics to this point.
Use keyword arguments in functions.

Review: Functions

Main points:

Define functions using the def keyword.
A function must be called before the code in it will run!
You will recognize function calls by the () at the end.

# This part is the function definition!
def say_hello():
    print("hello world!")

# This part is actually calling/running the function!
say_hello()

Review: Function Arguments

Provide an argument to a function when you need something small to vary.

Teaching tips:

This is a demo and recap, not an exercise.
They’ve seen this a few times, but make sure to recap the syntax again. We’re throwing a lot at them quickly.
Give other examples where parameters would be useful.

Talking points:

“A lot of times, you want to run the same code, but with a teeny tiny difference. Remember the function we looked at before which had the orders with the $5 shipping charge?”

Repl.it Note: The code here is:

def print_order(product):
  print("Thank you for ordering the " + product + ".")
  print("There will be a $5.00 shipping charge for this order.")

print_order("Trampoline")
print_order("Spider-Man Comic")
print_order("Hot Cheetos")

Multiple Parameters

Functions can have…

# No parameters
def add_2_and_3():
    x = 2 + 3
    print(x)

# One parameter
def add_2(x):
    print(x + 2)

# Multiple parameters
def add(x, y, z):
    print(x + y + z)

Discussion: Print vs Return

Why doesn’t this do anything?

def add(x, y, z):
    return x + y + z

add(1, 2, 3) # does nothing!

We Do: Review Exercises

Locally, let’s create a file called function_practice.py.

We’ll define a function named areBothEven.
It will accept two parameters: num1 and num2.
Inside the function, we’ll return True if num1 and num2 are both even but False if they are not.
We’ll test this with print(areBothEven(1, 4)), print(areBothEven(2, 4)), and print(areBothEven(2, 3)).

We Do: Another Review Exercise!

In our file, we’ll define another function named lightOrDark that takes the parameter hour.

If hour is greater than 24, the function will print “That’s not an hour in the day!” and return nothing.
If hour is less than 7 or greater than 17, the function will return “It’s dark outside!”
Otherwise, the function will return “It’s light outside!”
We’ll test this with print(lightOrDark(4)), print(lightOrDark(26)), and print(lightOrDark(10)).

Discussion: Arguments

Now, let’s make functions a little more sophisticated.

What do you think the following code does?

def multiply(x, y):
    print(x * y)

multiply(1, 2, 3) # Too many arguments! What happens?

What if we want all of these to work?

def multiply(x, y):
    print(x * y)

multiply(4, 5, 6)
multiply(4, 5)
multiply(4, 5, 2, 7, 3, 9)

Introducing `*args`

*args is a parameter that says “Put as many parameters as you’d like!”

Pronounced like a pirate - “arrrrghhhs!”
Known as positional arguments
The * at the beginning is what specifies the variable number of arguments

def multiply(*args):
    product = 1

    # We don't know the number of args, so we need a loop
    for num in args:
        product *= num
    print(product)

multiply(4, 5, 6) # Prints 120!

We Do: `*args`

Let’s create a local file for this lesson - args_practice.py.

We’ll write a function, sum_everything that takes any numbers of arguments and adds them together.
At the end, we’ll print out the sum.
Let’s try it with sum_everything(4, 5, 6) and sum_everything(6, 4, 5). The order doesn’t matter!
*args says “any number” - you can pass in none at all!

Teaching tips:

Do this with them, but make sure they have a file out and are following along. The more they practice writing this, the better.
See if they can guess what a line should be before you write it.
Since it’s a loop, the order of arguments doesn’t matter.
Once the code is written, show them a varying number of arguments being passed in. 1? 4? Without any arguments passed in at all (especially this one!).

Solution Code:

def sum_everything(*args):
    sum = 0

    # We don't know the number of args, so we need a loop
    for num in args:
        sum += num
    print(sum)

sum_everything(4, 5, 6) # Prints 15

Discussion: Often, Order Does Matter.

Let’s switch gears. Back to a set number of arguments!

Check this out:

def triple_divide(x, y, z):
    print(x / y / z)

triple_divide(1, 2, 10) # Prints 0.05

Without otherwise specifying, x is 1, y is 2, and z is 10.

What if we want x, the first parameter to get the value 10?
Is there a way to specify which argument goes to which parameter?

Forcing the Order

Here we’ve forced the order to be reversed from the default. In fact, we can specify any ordering we want by using the names of the parameters (keywords) when providing the argument values.

Take a moment to play around with the values until you really believe it!

Keyword Arguments (kwargs)

Using kwargs, odrer deons’t mtater:

Arguments are named according to their corresponding parameters.
Order doesn’t matter - Python will check the names and match them!
Values are assigned because the keyword argument and the parameter name match.

def triple_divide(x, y, z):
    print(x / y / z)

triple_divide(x=10, y=2, z=1)
# This runs 10 / 2 / 1, and prints 5
triple_divide(y=2, z=1, x=10)
# This ALSO runs 10 /  2 / 1, and prints 5.

Protip: Keep your parameter names simple and concise to prevent typos and misspellings!

Mix It Up…. but Not With Every Argument?

Fun fact: You can provide some args in order - positional - and some with keywords.

Undefined are assigned in sequential order.
Keywords have to come last! - then, in any order.

dinner(app="chicken wings", main_course="medium rare steak", drink="water", dessert="milkshake")
dinner("chicken wings", "water", dessert="milkshake", main_course="medium rare steak")

Talking points:

As you make the demo more and more complex, make sure to talk about what you’re doing.

Teaching tips:

This is a demo, so don’t worry about them following along (though it’s great if they do!).
First, recap and call the function with the kwargs in different orders: dinner(app="nachos", main_course="steak", drink="water", dessert="cake")
Then, talk about how you don’t necessarily need every keyword defined - but they’ll automatically be assigned in order. Demo that: dinner("water", "nachos", dessert="cake", main_course="steak") dinner("nachos", "water", dessert="steak", main_course="steak")
Then, show what happens if you put a kwarg first: dinner(main_course= "steak", "nachos", "water", dessert="cake")
What happens if you name two arguments the same? dinner(app="nachos", app="steak", app="water", dessert="cake")
End with correct syntax, so that’s freshest in their mind.

Repl.it Note: The code here is:

def dinner(drink, app, main_course, dessert):
  print("You're drinking", drink)
  print("You're snacking on", app)
  print("You're drinking", main_course)
  print("You're wrapping up with", dessert)

dinner(app="nachos", main_course="steak", drink="water", dessert="cake")

Quick Review

*args: Any number of arguments - even 0! - can be passed in.

def sum_everything(*args):
    sum = 0

    for num in args:
        sum += num
    print(sum)

sum_everything(4, 5, 6) # Prints 15

Keyword arguments (kwargs): Arguments can be passed in out of order.

def divide(first, second, third):
    print(first / second / third)

divide(first=10, second=2, third=1)
divide(second=2, third=1, first=10)

Discussion: Variable Numbers of Kwargs?

What if I go to Froyo? I need:

One argument spoon, to pick a spoon size.
A variable number of arguments for all the flavors of frozen yogurt I might eat!

def yogurt_land(*args)?

No! *args won’t work - we need to know which arg is the spoon.

def yogurt_land(spoon, froyo)?

No! We don’t know the number of froyo arguments.

Any ideas?

Introducing: `**kwargs`

The * in *args means: Any number of arguments.

Let’s add ** to our kwargs: **kwargs can take a variable number of arguments. Note the double **!

def yogurt_land(spoon, **kwargs):
  print(spoon)
  # We need a loop, because we don't know how many kwargs there are.
  for keyword, flavor in kwargs.items():
    # kwargs.items has the keyword and the value, which we're calling "flavor" in the loop.
    print("My", keyword, "is a", flavor)

# Like before, the unnamed arg has to come first!
yogurt_land("large!", first_froyo="vanilla", second_froyo="chocolate", third_froyo="banana")

We Do: 4 Froyos

Can we subtract one of the froyos?
Where is my 4th froyo?
What if I drop all my froyos on the ground? (No kwargs)
Can I skip the drink or spoon positional arguments?

Teaching tips:

This is a demo slide, not an exercise. Spend enough time to make sure they understand what’s going on, then move on.
Show changing the order and number of kwargs. Show having no kwargs. Trying giving all kwargs the same keyword. For each of these, talk about what’s happening and why. Try putting the kwargs in the definition first, and in the function call first. Remember not to mention dictionaries!
Show all variations with spoon: having no spoon, then giving the spoon the kwarg spoon, then putting the spoon at the end (and then in the middle) of the kwargs, then putting the spoon argument (with no spoon kwarg at other places). For each of these, talk about what’s happening and why.
End again with the correct syntax, so students remember what it should look like.

Repl.it Note: The repl.it has

def yogurt_land(drink, spoon, **kwargs):
  if spoon:
    print("Here is your spoon!")

  else:
    print("No spoon, no worries")

  print("Here is your", drink)

  for keyword, flavor in kwargs.items():
    print("My", keyword, "is a", flavor)


yogurt_land("water", "large", first_froyo="vanilla", second_froyo="chocolate", third_froyo="banana")

Quick Review of Useful Argument Types:

At this point, we have *args, kwargs and **kwargs:

# Args: Any number of arguments:
def multiply(*args):
    product = 1
    for num in args:
        product *= num

multiply(4, 5, 6)

# Kwargs: Named (keyword) arguments
def triple_divide(x, y, z):
    print(x / y / z)

triple_divide(x=10, y=2, z=1)

# **Kwargs: Any number of Kwargs
def my_froyo(spoon, **kwargs):
  for froyo, flavor in kwargs.items():
    print(froyo, "is a", flavor)

my_froyo("large!", froyo1="vanilla", froyo2="chocolate", froyo3="banana")

Discussion: Printing

print is a function! That’s why it has parentheses! - It’s built into Python, so you don’t have to define it. You can just use it.

When printing, commas automatically add spaces:

print("Hi!", "Vanilla,", "please.")

But since print is a function, too - do you think there’s anything we can do to change those spaces to something else?

# Hi!-Vanilla,-please,-but-chocolate-is-cool.
# Hi!-and-Vanilla,-and-please.

Print is AWESOME: Optional Parameters

Turns out…

print accepts an optional keyword argument: sep.

The sep value given will be used as a separator.

It’s optional! Without it, print by default uses a space, which is why you haven’t seen it.
This only applies when using commas.

print("Hi!", "Vanilla", "please,",  "but", "chocolate", "is", "cool.", sep="--")
# => Hi!--Vanilla,--please,--but--chocolate--is--cool.

Delicious Printing

We can replace name and dessert with your own name and favorite dessert. It’s a regular print!
sep can be any string, or even an icon (they’re made of strings - we’ll see later!) - but not an int.

Teaching tips:

This is just a demo! Show it working and check for understanding.
Change the value of sep to numbers, other strings, and even back to the default space. Then, leave it out again to remind them that this is a normal print statement. Then try putting sep first and show that it doesn’t work - it’s a kwarg, after all!
Point out the special character and how cool Python is, but don’t go into unicode or how it’s done. We can talk about that in unit 4.

Repl.it Note The code here is:

name = "Brandi"
dessert = "froyo"
print("Hello", "my", "name", "is", name, "and", "I", "enjoy", dessert, ":)")
print("Hello", "my", "name", "is", name, "and", "I", "enjoy", dessert, ":)", sep=" HELLO ")
print("Hello", "my", "name", "is", name, "and", "I", "enjoy", dessert, ":)", sep="🍧")

Quick Review

So far, we’ve learned:

*args:
- A variable number of function arguments.
kwargs:
- A set number of function arguments.
- Can be defined out of order
**kwargs:
- Any number of positional arguments.
sep in print.

There’s one more: Optional parameters.

Optional Parameters with Default Values

This idea exists in programming - you’ve already seen it!

The default value for sep in print is " ". You don’t need to include it.

This makes it optional! Optional parameters have default values, so you don’t need to include them.

Only include them if you want to change them!


# Here, `sep` is optional to include. It defaults to a space " ".
print("Hello", "my", "name", "is", name, "and", "I", "enjoy", dessert, ":)")

# But we can include it, if we want, and `sep` will use our value instead of the space.
print("Hello", "my", "name", "is", name, "and", "I", "enjoy", dessert, ":)", sep=" HELLO ")

Default parameters are in the function declaration.

They’re there if you don’t include a value.

Any Functions: Optional Parameters with Default Values

These can be added to any functions.

Here, c has a default of 20. We don’t need to include it!

# Optional parameters: Default values are only used if needed.
def my_func(a, b, c=20):
    print(a + b + c)
my_func(1, 2)
# Uses the default! Prints 23.
my_func(1, 2, 4)
# Overrides the default! Prints 7.

Partner Exercise: Poke At It!

Pair up! Choose a driver and a navigator.

In your local file, write a function, print_food that has four optional parameters (all with defaults of your choice): favorite_food, lunch_today, lunch_yesterday, and breakfast.

print_food should print out each of these.

Call this with a couple different arguments:

No arguments.
All arguments - a regular function call.
2 keyword arguments. Give all four arguments, but use a keyword for lunch_yesterday and breakfast.
All keyword arguments - out of order.

Partner Exercise: Keep Poking!

Change roles!

Underneath print_food, rewrite it, twice.

First, write print_food_args, using *args as the parameter. Start the function by printing args, so you can see what’s going on. Then, print the values you pass in.

Then, write print_food_kwargs, using **kwargs as the parameter. Start the function by printing kwargs, so you can see what’s going on. Then, as above, print the values you pass in.

Summary + Q&A

*args:
- A variable number of function arguments.
- Taken in any order.
- def multiply(*args):
kwargs:
- A set number of function arguments.
- Can be defined out of order
- my_func(a=1, b=2, c=3)
**kwargs: - Any number of positional arguments.
- def froyo(*kwargs)
sep in print.
Optional parameters:
- Default values in the function declaration
- def my_func(a=10, b=15, c=20)

Python Programming: Advanced Function Arguments

Lesson Objectives

Review: Functions

Review: Function Arguments

Multiple Parameters

Discussion: Print vs Return

We Do: Review Exercises

We Do: Another Review Exercise!

Discussion: Arguments

Introducing *args

We Do: *args

Discussion: Often, Order Does Matter.

Forcing the Order

Keyword Arguments (kwargs)

Mix It Up…. but Not With Every Argument?

Quick Review

Discussion: Variable Numbers of Kwargs?

Introducing: **kwargs

We Do: 4 Froyos

Quick Review of Useful Argument Types:

Discussion: Printing

Print is AWESOME: Optional Parameters

Delicious Printing

Quick Review

Optional Parameters with Default Values

Any Functions: Optional Parameters with Default Values

Partner Exercise: Poke At It!

Partner Exercise: Keep Poking!

Summary + Q&A

Additional Resources

Introducing `*args`

We Do: `*args`

Introducing: `**kwargs`