Question

I searched the site but it seems like this exercise from the book Learn Python the Hard way wasn’t covered in previous questions.

I have this exercise:

1)Find all the places where a string is put inside a string. There are four places.
2)Are you sure there’s only four places? How do you know? Maybe I like lying.

x = "There are %d types of people." % 10
binary = "binary"
do_not = "don't"
y = "Those who know %s and those who %s." % (binary, do_not) #two strings inside a string, count is 2

print x
print y

print "I said: %r." % x #here, count is 3
print "I also said: '%s'." % y #here, count is 4

hilarious = False
joke_evaluation = "Isn't that joke so funny?! %r"

print joke_evaluation % hilarious

w = "This is the left side of..."
e = "a string with a right side."

print w + e

I tried to solve it and commented the lines where I found a string inside a string(found4). But, the author’s second questions makes me worry I haven’t found all. Have I miss something? If yes, can you please tell me what?

Answer 1

If you interpret “putting a string inside a string” as “using a format character with a string argument”, you correctly identified all four occurrences. It may be more helpful to put away the useless instructions and explain what happens. For more information, have a look at the offical documentation for formatting characters. I’ll try to include some helpful exercises without trick questions.

x = "There are %d types of people." % 10

%d is the formatting character for a Signed integer decimal (what most people think of when hearing a “number”). 10 is expressed in decimal and inserted into the string, resulting in the string There are 10 types of people..

Excercise: What would "7+6: %d" % (7+6) result in? Test it in your Python shell.
Excercise: What would "0x12: %d" % 0x12 result in? Test it in your Python shell.
Hint: A prefix of 0x means the following number is hexadecimal, i.e. base 16 instead of 10.

binary = "binary"
do_not = "don't"
y = "Those who know %s and those who %s." % (binary, do_not)

%s inserts the string representation of a value. This example goes to show that if there is more than one %d, %s or so in a format string, we need to give that number of arguments, in a tuple or list. The difference between a tuple (round, braces) and a list [square, braces] is that a tuple is immutable(i.e. can’t be changed), but a list can.

Excercise: Given the tuple x = ('world', 'Hello'), construct a tuple y with the correct order of words. Dive Into Python’s introduction of tuples will help you understand the basics of tuples.

print "I said: %r." % x

%r is the representation of a value, ideally something you could enter into a Python shell. You can get the same representation with the repr function. For example, repr("a") == "'a'", whereas str(a) == "a". For most types except string, the result of repr and str (or %r and %s in format strings) is the same.

print w + e

This concatenates two strings (i.e. puts one after the other). Note that the + means something else (addition) if both arguments are numbers.

Excercise: What is the result of "3" + "4"? Test it in your Python shell.
Excercise: Given x = "3"; y = "4", print out the sum 7 (Hint).