4장 Choose with if
Table of contents
- Comment with
- Continue Lines with \
- Compare with if, elif, and else
- What Is True?
- Do Multiple Comparisons with in
- New: I Am the Walrus
- Coming Up
- Things to Do
- Choose with if.
- Comment with #
- Continue Lines with \
- Compare with if, elif, and else
- What Is True?
- Do Multiple Comparisons with in
- New: I Am the Walrus
- Coming Up
- Things to Do
- Choose with if.
CHAPTER 4 Choose with if
If you can keep your head when all about you
Are losing theirs and blaming it on you, ...
—Rudyard Kipling, If—
In the previous chapters, you’ve seen many examples of data but haven’t done much
with them. Most of the code examples used the interactive interpreter and were short.
In this chapter, you learn how to structure Python code, not just data.
Many computer languages use characters such as curly braces ({ and }) or keywords
such as begin and end to mark off sections of code. In those languages, it’s good prac‐
tice to use consistent indentation to make your program more readable for yourself
and others. There are even tools to make your code line up nicely.
When he was designing the language that became Python, Guido van Rossum deci‐
ded that the indentation itself was enough to define a program’s structure, and avoi‐
ded typing all those parentheses and curly braces. Python is unusual in this use of
white space to define program structure. It’s one of the first aspects that newcomers
notice, and it can seem odd to those who have experience with other languages. It
turns out that after writing Python for a little while, it feels natural, and you stop
noticing it. You even get used to doing more while typing less.
Our initial code examples have been one-liners. Let’s first see how to make comments
and multiple-line commands.
Comment with
A comment is a piece of text in your program that is ignored by the Python inter‐
preter. You might use comments to clarify nearby Python code, make notes to your‐
self to fix something someday, or for whatever purpose you like. You mark a
Choose with if | 53
1 Like that eight-legged green thing that’s right behind you!
2 Please don’t call it. It might come back.
comment by using the # character; everything from that point on to the end of the
current line is part of the comment. You’ll usually see a comment on a line by itself, as
shown here:
>>> # 60 sec/min * 60 min/hr * 24 hr/day
>>> seconds_per_day = 86400
Or, on the same line as the code it’s commenting:
>>> seconds_per_day = 86400 # 60 sec/min * 60 min/hr * 24 hr/day
The # character has many names: hash, sharp, pound, or the sinister-sounding octo‐
thorpe.^1 Whatever you call it,^2 its effect lasts only to the end of the line on which it
appears.
Python does not have a multiline comment. You need to explicitly begin each com‐
ment line or section with a #:
>>> # I can say anything here, even if Python doesn't like it,
... # because I'm protected by the awesome
... # octothorpe.
>>>
However, if it’s in a text string, the mighty octothorpe reverts back to its role as a plain
old # character:
>>> print ("No comment: quotes make the # harmless.")
No comment: quotes make the # harmless.
Continue Lines with \
Programs are more readable when lines are reasonably short. The recommended (not
required) maximum line length is 80 characters. If you can’t say everything you want
to say in that length, you can use the continuation character: \ (backslash). Just put \
at the end of a line, and Python will suddenly act as though you’re still on the same
line.
For example, if I wanted to add the first five digits, I could do it a line at a time:
>>> sum = 0
>>> sum += 1
>>> sum += 2
>>> sum += 3
>>> sum += 4
>>> sum
10
54
54 | Chapter 4: Choose with if
Or, I could do it in one step, using the continuation character:
>>> sum = 1 + \
... 2 + \
... 3 + \
... 4
>>> sum
10
If we skipped the backslash in the middle of an expression, we’d get an exception:
>>> sum = 1 +
File "<stdin>", line 1
sum = 1 +
^
SyntaxError: invalid syntax
Here’s a little trick—if you’re in the middle of paired parentheses (or square or curly
brackets), Python doesn’t squawk about line endings:
>>> sum = (
... 1 +
... 2 +
... 3 +
... 4)
>>>
>>> sum
10
You’ll also see in Chapter 5 that paired triple quotes let you make multiline strings.
Compare with if, elif, and else
Now, we finally take our first step into the code structures that weave data into pro‐
grams. Our first example is this tiny Python program that checks the value of the
boolean variable disaster and prints an appropriate comment:
>>> disaster = True
>>> if disaster:
... print ("Woe!")
... else :
... print ("Whee!")
...
Woe!
>>>
The if and else lines are Python statements that check whether a condition (here,
the value of disaster) is a boolean True value, or can be evaluated as True. Remem‐
ber, print() is Python’s built-in function to print things, normally to your screen.
Compare with if, elif, and else | 55
If you’ve programmed in other languages, note that you don’t need
parentheses for the if test. For example, don’t say something such
as if (disaster == True) (the equality operator == is described
in a few paragraphs). You do need the colon (:) at the end. If, like
me, you forget to type the colon at times, Python will display an
error message.
Each print() line is indented under its test. I used four spaces to indent each subsec‐
tion. Although you can use any indentation you like, Python expects you to be consis‐
tent with code within a section—the lines need to be indented the same amount, lined
up on the left. The recommended style, called PEP-8, is to use four spaces. Don’t use
tabs, or mix tabs and spaces; it messes up the indent count.
We did a number of things here, which I explain more fully as the chapter progresses:
- Assigned the boolean value True to the variable named disaster
- Performed a conditional comparison by using if and else, executing different code depending on the value of disaster
- Called the print() function to print some text
You can have tests within tests, as many levels deep as needed:
>>> furry = True
>>> large = True
>>> if furry:
... if large:
... print ("It's a yeti.")
... else :
... print ("It's a cat!")
... else :
... if large:
... print ("It's a whale!")
... else :
... print ("It's a human. Or a hairless cat.")
...
It's a yeti.
In Python, indentation determines how the if and else sections are paired. Our first
test was to check furry. Because furry is True, Python goes to the indented if large
test. Because we had set large to True, if large is evaluated as True, and the
following else line is ignored. This makes Python run the line indented under if
large: and print It’s a yeti.
If there are more than two possibilities to test, use if for the first, elif (meaning else
if) for the middle ones, and else for the last:
| **56 | Chapter 4: Choose with if** |
>>> color = "mauve"
>>> if color == "red":
... print ("It's a tomato")
... elif color == "green":
... print ("It's a green pepper")
... elif color == "bee purple":
... print ("I don't know what it is, but only bees can see it")
... else :
... print ("I've never heard of the color", color)
...
I've never heard of the color mauve
In the preceding example, we tested for equality by using the == operator. Here are
Python’s comparison operators:
equality ==
inequality !=
less than <
less than or equal <=
greater than >
greater than or equal >=
These return the boolean values True or False. Let’s see how these all work, but first,
assign a value to x:
>>> x = 7
Now, let’s try some tests:
>>> x == 5
False
>>> x == 7
True
>>> 5 < x
True
>>> x < 10
True
Note that two equals signs (==) are used to test equality; remember, a single equals
sign (=) is what you use to assign a value to a variable.
If you need to make multiple comparisons at the same time, you use the logical (or
boolean) operators and, or, and not to determine the final boolean result.
Logical operators have lower precedence than the chunks of code that they’re compar‐
ing. This means that the chunks are calculated first, and then compared. In this
example, because we set x to 7 , 5 < x is calculated to be True and x < 10 is also True,
so we finally end up with True and True:
Compare with if, elif, and else | 57
>>> 5 < x and x < 10
True
As “Precedence” on page 43 points out, the easiest way to avoid confusion about
precedence is to add parentheses:
>>> (5 < x) and (x < 10)
True
Here are some other tests:
>>> 5 < x or x < 10
True
>>> 5 < x and x > 10
False
>>> 5 < x and not x > 10
True
If you’re and-ing multiple comparisons with one variable, Python lets you do this:
>>> 5 < x < 10
True
It’s the same as 5 < x and x < 10. You can also write longer comparisons:
>>> 5 < x < 10 < 999
True
What Is True?
What if the element we’re checking isn’t a boolean? What does Python consider True
and False?
A false value doesn’t necessarily need to explicitly be a boolean False. For example,
these are all considered False:
boolean False null None zero integer 0 zero float 0.0 empty string ‘’ empty list [] empty tuple () empty dict {} empty set set()
__bool__()메서드로 확인 가능
Anything else is considered True. Python programs use these definitions of “truthi‐
ness” and “falsiness” to check for empty data structures as well as False conditions:
| **58 | Chapter 4: Choose with if** |
>>> some_list = []
>>> if some_list:
... print ("There's something in here")
... else :
... print ("Hey, it's empty!")
Hey, it's empty!
If what you’re testing is an expression rather than a simple variable, Python evaluates
the expression and returns a boolean result. So, if you type:
if color == "red":
Python evaluates color == “red”. In our earlier example, we assigned the string
“mauve” to color, so color == “red” is False, and Python moves on to the next test:
elif color == "green":
Do Multiple Comparisons with in
Suppose that you have a letter and want to know whether it’s a vowel. One way would
be to write a long if statement:
>>> letter = 'o'
>>> if letter == 'a' or letter == 'e' or letter == 'i' \
... or letter == 'o' or letter == 'u':
... print (letter, 'is a vowel')
... else :
... print (letter, 'is not a vowel')
...
o is a vowel
>>>
Whenever you need to make a lot of comparisons like that, separated by or, use
Python’s membership operator in, instead. Here’s how to check vowel-ness more
Pythonically, using in with a string made of vowel characters:
>>> vowels = 'aeiou'
>>> letter = 'o'
>>> letter in vowels
True
>>> if letter in vowels:
... print (letter, 'is a vowel')
...
o is a vowel
Here’s a preview of how to use in with some data types that you’ll read about in detail
in the next few chapters:
>>> letter = 'o'
>>> vowel_set = {'a', 'e', 'i', 'o', 'u'}
>>> letter in vowel_set
Do Multiple Comparisons with in | 59
True
>>> vowel_list = ['a', 'e', 'i', 'o', 'u']
>>> letter in vowel_list
True
>>> vowel_tuple = ('a', 'e', 'i', 'o', 'u')
>>> letter in vowel_tuple
True
>>> vowel_dict = {'a': 'apple', 'e': 'elephant',
... 'i': 'impala', 'o': 'ocelot', 'u': 'unicorn'}
>>> letter in vowel_dict
True
>>> vowel_string = "aeiou"
>>> letter in vowel_string
True
For the dictionary, in looks at the keys (the lefthand side of the :) instead of their
values.
New: I Am the Walrus
Arriving in Python 3.8 is the walrus operator, which looks like this:
name := expression
See the walrus? (Like a smiley, but tuskier.)
Normally, an assignment and test take two steps:
>>> tweet_limit = 280
>>> tweet_string = "Blah" * 50
>>> diff = tweet_limit - len(tweet_string)
>>> if diff >= 0:
... print ("A fitting tweet")
... else :
... print ("Went over by", abs(diff))
...
A fitting tweet
With our new tusk power (aka assignment expressions) we can combine these into
one step:
>>> tweet_limit = 280
>>> tweet_string = "Blah" * 50
>>> if diff := tweet_limit - len(tweet_string) >= 0:
... print ("A fitting tweet")
... else :
... print ("Went over by", abs(diff))
...
A fitting tweet
The walrus also gets on swimmingly with for and while, which we look at in Chapter 6.
이렇게 if, else에서 변수에 값을 할당할 때는 변수 = 값 if 조건문 else 값 형식으로 축약할 수 있으며 이런 문법을 조건부 표현식(conditional expression)이라고 부릅니다.
>>> x = 5
>>> if x == 10:
... y = x
... else:
... y = 0
...
>>> y
0
동일 결과
>>> x = 5
>>> y = x if x == 10 else 0
>>> y
0
Coming Up
Play with strings, and meet interesting characters.
Things to Do
4.1 Choose a number between 1 and 10 and assign it to the variable secret. Then,
select another number between 1 and 10 and assign it to the variable guess. Next,
write the conditional tests (if, else, and elif) to print the string ‘too low’ if guess
is less than secret, ‘too high’ if greater than secret, and ‘just right’ if equal to
secret.
4.2 Assign True or False to the variables small and green. Write some if/else state‐
ments to print which of these matches those choices: cherry, pea, watermelon,
pumpkin.