Writing Functions

Overview

Teaching: 10 min
Exercises: 15 min

Questions

• How can I create my own functions?

Objectives

• Explain and identify the difference between function definition and function call.

• Write a function that takes a small, fixed number of arguments and produces a single result.

Break programs down into functions to make them easier to understand.

• Human beings can only keep a few items in working memory at a time.
• Understand larger/more complicated ideas by understanding and combining pieces.
• Functions serve the same purpose in programs.
  • Encapsulate complexity so that we can treat it as a single “thing”.
• Also enables re-use.
  • Write one time, use many times.

Define a function using def with a name, parameters, and a block of code.

• Begin the definition of a new function with def.
• Followed by the name of the function.
  • Must obey the same rules as variable names.
• Then parameters in parentheses.
  • Empty parentheses if the function doesn’t take any inputs.
  • We will discuss this in detail in a moment.
• Then a colon.
• Then an indented block of code.

def print_greeting():
    print('Hello!')

Defining a function does not run it.

• Defining a function does not run it.
  • Like assigning a value to a variable.
• Must call the function to execute the code it contains.
• The commands for the function are read and stored after the def block, but not actually executed until the function is called later on.
  • Imagine getting a recipe card and keeping it in your kitchen. You can cook it anytime, but you haven’t completed any of the steps until you start that cooking process.
  • This means that Python won’t complain about problems until you call the function. More specifically, just because the definition of a function runs without error doesn’t mean that there won’t be errors when it executes later.

print_greeting()

Hello!

Arguments in call are matched to parameters in definition.

• Functions are most useful when they can operate on different data.
• Specify parameters when defining a function.
  • These become variables when the function is executed.
  • Are assigned the arguments in the call (i.e., the values passed to the function).

def print_date(year, month, day):
    joined = str(year) + '/' + str(month) + '/' + str(day)
    print(joined)

print_date(1871, 3, 19)

1871/3/19

Functions may return a result to their caller using return.

• Use return ... to give a value back to the caller.
• May occur anywhere in the function.
• But functions are easier to understand if return occurs:
  • At the start to handle special cases.
  • At the very end, with a final result.
• Remember: every function returns something.
• A function that doesn’t explicitly return a value automatically returns None.

Definition and Use

What does the following program print? Edwidge Danticat gave a lecture yesterday here.

def report(author):
    return 'author was' + author

report('Edwidge Danticat')

Solution

author is Edwidge Danticat

Calling by Name

What does this short program print?

def add_https(url):
    joined = "https:" + url
    return joined

my_url = "//www.loc.gov/item/2006627699/"
correct_url = add_https(my_url)
print(correct_url)

Using the .format() method on strings

We’ve been using the + to add strings together. We can also use format to create strings.

genre = "Fiction"
subject = "Haitian Americans--{}".format(genre)
print(subject)

[SLIDE]

Rewrite the add_https() function to use .format() instead of concatenating strings.

** Answer **

def add_https(url):
    joined = "https:{}".format(url)
    return joined

my_url = "//www.loc.gov/item/2006627699/"
correct_url = add_https(my_url)
print(correct_url)

Key Points

• Break programs down into functions to make them easier to understand.

• Define a function using def with a name, parameters, and a block of code.

• Defining a function does not run it.

• Arguments in call are matched to parameters in definition.

• Functions may return a result to their caller using return.