Chapter 6: Dictionaries and Sets 1
Dictionaries and Sets
6.1 Introduction
6.1 Q1: Which of the following statements is false?
a. Lists and tuples are built-in sequence collections.
b. Dictionaries, strings and sets are built-in non-sequence collections.
c. A dictionary is an unordered collection which stores key–value pairs that map
immutable keys to values, just as a conventional dictionary maps words to defi-
nitions.
d. A set is an unordered collection of unique immutable elements.
6.2 Dictionaries
6.2 Q1: Which of the following statements a), b) or c) is false?
a. A dictionary associates keys with values. Each key maps to a specific value.
b. A dictionary’s keys must be mutable (such as strings, numbers or tuples) and
unique (that is, no duplicates).
c. Multiple keys can have the same value, such as two different inventory codes
that have the same quantity in stock.
d. All of the above statements are true.
6.2.1 Creating a Dictionary
6.2 Q2: Which of the following statements is false?
a. You can create a dictionary by enclosing in curly braces, {}, a comma-separated
list of key–value pairs, each of the form key: value.
b. You can create an empty dictionary with {}.
c. The following statement creates a dictionary with the country-name keys
‘Finland’, ‘South Africa’ and ‘Nepal’ and their corresponding Internet
country code values ‘fi’, ‘za’ and ‘np’:
country_codes = {‘Finland’, ‘fi’, ‘South Africa’, ‘za’,
‘Nepal’, ‘np’}
d. When you output a dictionary, its comma-separated list of key–value pairs is
always enclosed in curly braces.
2 Chapter 6: Dictionaries and Sets
© Copyright 2020 by Pearson Education, Inc. All Rights Reserved.
Answer: c. Actually, the following code creates a dictionary with the coun-
try-name keys ‘Finland’, ‘South Africa’ and ‘Nepal’ and their corre-
sponding Internet country code values ‘fi’, ‘za’ and ‘np’:
country_codes = {‘Finland’: ‘fi’, ‘South Africa’: ‘za’,
‘Nepal’: ‘np’}
6.2 Q3: Which of the following statements is false?
a. When you pass a dictionary to built-in function len, the function returns the
number of key–value pairs in the dictionary.
b. You can use a dictionary as a condition to determine if it’s empty—a non-empty
dictionary evaluates to True.
c. An empty dictionary evaluates to False.
d. Method clean deletes the dictionary’s key–value pairs:
6.2.2 Iterating through a Dictionary
6.2 Q4: Which of the following statements a), b) and c) is false?
a. The following dictionary maps month-name strings to int values representing
the numbers of days in the corresponding month:
days_per_month = {‘January’: 31, ‘February’: 28, ‘March’: 31}
b. Multiple keys in a dictionary can have the same value.
c. The following for statement iterates through dictionary days_per_month’s
key–value pairs. Dictionary method items returns each key–value pair as a tuple,
which is unpacked into the target variables month and days:
for month, days in days_per_month.items():
print(f‘{month} has {days} days’)
d. All of the above statements are true.
6.2.3 Basic Dictionary Operations
6.2 Q5: Which of the following statements a), b) or c) is false?
a. The following code creates the dictionary roman_numerals, which maps ro-
man numerals to their integer equivalents (the value for ‘X’ is intentionally
wrong):
roman_numerals = {‘I’: 1, ‘II’: 2, ‘III’: 3, ‘V’: 5, ‘X’: 100}
Chapter 6: Dictionaries and Sets 3
b. The following code gets the value associated with the key ‘V’ in Part (a):
roman_numerals[‘V’]
c. You can update a key’s associated value in an assignment statement. The fol-
lowing code replaces the incorrect value associated with the key ‘X’ in Part (a):
roman_numerals[‘X’] = 10
d. All of the above statements are true.
6.2 Q6: Which of the following statements a), b) or c) is false?
a. Assigning a value to a nonexistent key inserts the key–value pair in the diction-
ary.
b. String keys are case insensitive.
c. Assigning a value to a nonexistent key could be a logic error.
d. All of the above statements are true.
6.2.4 Dictionary Methods keys and values
6.2 Q7: Which of the following statements is false?
a. Dictionary methods keys and values can be used to iterate through a diction-
ary’s keys or values, respectively.
b. Dictionary methods items, keys and values each return a view of a diction-
ary’s data.
c. When you iterate over a view, it “sees” a copy of the dictionary’s current con-
tents.
d. You should not modify a dictionary while iterating through a view. According
to the Python Standard Library documentation, either you’ll get a RuntimeError
or the loop might not process all of the view’s values.
6.2.5 Dictionary Comparisons
No questions.
6.2.6 Example: Dictionary of Student Grades
6.2 Q8: Which of the following statements a), b) or c) is false?
4 Chapter 6: Dictionaries and Sets
a. The comparison operators == and != can be used to determine whether two
dictionaries have identical or different contents.
b. An equals (==) comparison evaluates to True if both dictionaries have the same
key–value pairs, regardless of the order in which those pairs were added to each
dictionary.
c. In the following dictionary, the keys are lists of three integer student grades
and the values are strings representing the students’ names.
grade_book = {
‘Susan’: [92, 85, 100],
‘Eduardo’: [83, 95, 79],
‘Azizi’: [91, 89, 82],
‘Pantipa’: [97, 91, 92]
}
d. All of the above statements are true.
6.2.7 Example: Word Counts
6.2 Q9: Which of the following statements a), b) or c) is false?
a. The following code builds a dictionary to count the number of occurrences of
each word in a string named text—the dictionary’s keys will be the unique
words, and its values will be integer counts of how many times each word appears
in text:
word_counts = {}
for word in text.split():
if word in word_counts:
word_counts[word] += 1
else:
word_counts[word] = 1
b. Breaking a string into words is known as tokenizing the string.
c. Python automatically concatenates strings separated by whitespace in paren-
theses.
d. All of the above statements are true.
6.2 Q10: Which of the following statements about the following code is false?
word_counts = {}
Chapter 6: Dictionaries and Sets 5
for word in text.split():
if word in word_counts:
word_counts[word] += 1
else:
word_counts[word] = 1
a. The expression text.split() tokenizes text by calling string method split,
which separates the words using the method’s delimiter string argument—if you
do not provide an argument, split uses a space.
b. Method split returns a list of tokens (that is, the words in text).
c. The expression word_counts[word] += 1 inserts a new key-value pair in the
dictionary.
d. All of the above statements are true.
6.2.8 Dictionary Method update
6.2 Q11: Which of the following statements a), b) or c) is false?
a. The following call to dictionary method update receives a dictionary contain-
ing one key–value pair to insert or update:
country_codes.update({‘South Africa’: ‘za’})
b. Method update can convert keyword arguments into key–value pairs to insert.
The following call converts the parameter name Australia into the string key
‘Australia’ and associates the incorrect value ‘ar’ with that key:
country_codes.update(Australia=‘ar’)
c. The snippet in Part (b) provided an incorrect country code (‘ar’) for Aus-
tralia. The following code corrects this by using a keyword argument to update
the value associated with ‘Australia’:
country_codes.update(Australia=‘au’)
d. All of the above statements are true.
6.2.9 Dictionary Comprehensions
6.2 Q12: Which of the following statements a), b) or c) is false?
a. Dictionary comprehensions provide a convenient notation for quickly generat-
ing dictionaries, often by mapping one dictionary to another.
b. The following code, which has a dictionary with unique values, reverses the
key–value pairs, so the values become the keys and vice versa:
6 Chapter 6: Dictionaries and Sets
months = {‘January’: 1, ‘February’: 2, ‘March’: 3}
months2 = {number: name for name, number in months.items()}
c. If the values in Part (b) were not unique, then the code would generate a
ValueError.
d. All of the above statements are true.
6.3 Sets
6.3 Q1: Which of the following statements is false?
a. A set is an unordered collection of unique values.
b. Sets may contain only immutable objects, like strings, ints, floats and tuples
that contain only immutable elements.
c. Sets are iterable, so they are sequences and they support indexing and slicing
with square brackets, [].
d. Dictionaries do not support slicing.
6.3 Q2: Which of the following statements a), b) or c) is false?
a. The following code creates a set of strings named colors:
colors = {‘red’, ‘orange’, ‘yellow’, ‘green’, ‘red’,
‘blue’}
b. Duplicate elimination is automatic when creating a set.
c. Sets are ordered, so you can write code that depends on the order of their ele-
ments.
d. All of the above statements are true.
6.3 Q3: Which of the following statements is false?
a. You can determine the number of items in a set with the built-in len function.
b. You can check whether a set contains a particular value using the in and not
in operators.
c. Sets are not iterable, so you cannot process each set element with a for loop.
Chapter 6: Dictionaries and Sets 7
d. Sets are unordered, so there’s no significance to the iteration order.
6.3 Q4: Consider the following session:
In [1]: numbers = list(range(10)) + list(range(5))
In [2]: numbers
Out[2]: [0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 0, 1, 2, 3, 4]
In [3]: set(numbers)
Out[3]: ???
Which of the following would be displayed where we wrote ??? in Out[3]?
a. [0, 1, 2, 3, 4, 5, 6, 7, 8, 9]
b. (0, 1, 2, 3, 4, 5, 6, 7, 8, 9)
c. {0, 1, 2, 3, 4, 5, 6, 7, 8, 9}
d. {[0, 1, 2, 3, 4, 5, 6, 7, 8, 9]}
6.3 Q5: Which of the following statements a), b) or c) is false?
a. Sets are immutable, so sets can have other sets as elements.
b. A frozenset is an immutable set—it cannot be modified after you create it, so a
set can contain frozensets as elements.
c. The built-in function frozenset creates a frozenset from any iterable.
d. All of the above statements are true.
6.3.1 Comparing Sets
6.3 Q6: Which of the following statements is false?
a. The <= operator tests whether the set to its left is an improper subset of the one
to its right—that is, all the elements in the left operand are in the right operand,
and the sets might be equal.
b. You also can check for an improper subset with the set method issubset.
c. You can check for an improper superset with the set method issuperset.
d. The argument to issubset or issuperset must be a set.
8 Chapter 6: Dictionaries and Sets
© Copyright 2020 by Pearson Education, Inc. All Rights Reserved.
Answer: d. Actually, the argument to issubset or issuperset can be any
iterable. When either of these methods receives a non-set iterable argu-
ment, it first converts the iterable to a set, then performs the operation.
6.3.2 Mathematical Set Operations
6.3 Q7: Which of the following statements is false?
a. The union of two sets is a set consisting of all the unique elements from both
sets.
b. You can calculate the union with the | operator or with the set type’s union
method.
c. The operands of the binary set operators, like |, must both be any iterable.
d. The set methods may receive any iterable object as an argument.
6.3 Q8: Which of the following statements a), b) or c) is false?
a. You can calculate the intersection with the & operator or with the set type’s
intersection method.
b. The difference between two sets is a set consisting of the elements in the left
operand that are not in the right operand.
c. You can calculate the difference with the – operator or with the a set’s differ-
ence method.
d. All of the above statements are true.
6.3 Q9: What values are actually displayed where we’ve inserted ??? in Out[1]
and Out[2]:
In [1]: {1, 3, 5}.isdisjoint({2, 4, 6})
Out[1]: ???
In [2]: {1, 3, 5}.isdisjoint({4, 6, 1})
Out[2]: ???
a. False, False
b. False, True
c. True, False
d. True, True
10 Chapter 6: Dictionaries and Sets
d. All of the above statements are true.
6.4.1 How Dynamic Visualization Works
6.4 Q2: Which of the following statements a), b) or c) is false?
a. For each animation frame, a Matplotlib FuncAnimation calls a function that
you define to specify how to change the plot.
b. Displaying animation frames to the screen is relatively fast compared to the die
rolls, which occur at the computer’s CPU speeds.
c. If we roll only one die per animation frame, we won’t be able to run a large
number of rolls in a reasonable amount of time. You can increase the execution
speed of the simulation by rolling the die more times per animation frame.
d. All of the above statements are true.
6.4.2 Implementing a Dynamic Visualization
6.4 Q3: Which of the following statements about FuncAnimation is false?
a. FuncAnimation has two required arguments—the Figure object in which to
display the animation, and a function to call once per animation frame.
b. The FuncAnimation argument interval is the number of milliseconds be-
tween animation frames (the default is 200).
c. The FuncAnimation argument fargs (short for “function arguments”) is a tu-
ple of other arguments to pass to the function you specify in FuncAnimation’s
second argument.
d. All of the above statements are true.