Python Dictionaries

What is a dictionary in Python?

• A collection of ordered items

• A mapping from keys to values

• A sequence of characters

• A type of loop

Answer explanation

A dictionary in Python is a data structure that stores data as key-value pairs, making it a mapping from keys to values. This allows for efficient data retrieval and organization, distinguishing it from ordered collections or sequences.

Which of the following is a correct way to create an empty dictionary?

• numbers = ()

• numbers = []

• numbers = ''

• numbers = {}

Answer explanation

The correct way to create an empty dictionary in Python is using curly braces: numbers = {}. The other options create different data types: () is a tuple, [] is a list, and '' is a string.

What will be the output of len({'zero': 0, 'one': 1, 'two': 2})?

• 1

• 0

• 2

• 3

Answer explanation

The function len() returns the number of key-value pairs in a dictionary. The given dictionary {'zero': 0, 'one': 1, 'two': 2} has three pairs: 'zero', 'one', and 'two'. Therefore, the output is 3.

Which operator is used to check if a key exists in a dictionary?

• !=

• not in

• ==

• in

Answer explanation

The 'in' operator is used to check if a key exists in a dictionary. It returns True if the key is found, making it the correct choice for this question.

What is the result of numbers.get('three', 0) if 'three' is not a key in the dictionary?

• KeyError

• None

• 3

• 0

Answer explanation

The method numbers.get('three', 0) returns the second argument (0) when 'three' is not a key in the dictionary. Therefore, the result is 0.

What does the method values() do in a dictionary?

• Returns a list of keys

• Returns a list of values

• Deletes a key-value pair

• Adds a new key-value pair

Answer explanation

The method values() in a dictionary returns a list of all the values stored in the dictionary. Therefore, the correct answer is 'Returns a list of values', as it specifically retrieves the values associated with the keys.

What is the result of the expression:
'one' in {'zero': 0, 'one': 1, 'two': 2}

• None

• True

• KeyError

• False

Answer explanation

The expression checks if 'one' is a key in the given dictionary. Since 'one' is indeed a key in {'zero': 0, 'one': 1, 'two': 2}, the result is True.

What is filtering in the context of programming?

• Formatting output

• Creating a dictionary

• Sorting a list of numbers

• Looping through a sequence and selecting or omitting elements

Answer explanation

Filtering in programming refers to looping through a sequence (like a list) and selecting or omitting elements based on certain criteria, making it the correct choice. The other options do not accurately describe filtering.

What is the output of the following code:
d={'a': 1, 'b': 2}
['a'] = 3
d['a']

• KeyError

• 1

• 3

• 2

Answer explanation

The line ['a'] = 3 is invalid syntax and will raise a SyntaxError. However, if we assume it meant d['a'] = 3, then d['a'] would be updated to 3. Thus, d['a'] outputs 3.

What is the purpose of the get() method in a dictionary?

• To check if a key exists

• To retrieve a value for a given key (with an optional default)

• To add a new key-value pair

• To remove a key-value pair

Answer explanation

The get() method in a dictionary is used to retrieve a value for a specified key, allowing for an optional default value if the key does not exist. This makes it a safe way to access dictionary values.