Dictionary creation in Python - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a dictionary in Python?

A dictionary is a collection of key-value pairs where each key is unique and used to access its corresponding value.

beginner

How do you create an empty dictionary in Python?

You create an empty dictionary using curly braces: {} or the dict() function.

beginner

Create a dictionary with keys 'name' and 'age' and values 'Alice' and 30.

Use {'name': 'Alice', 'age': 30} to create the dictionary.

intermediate

Can dictionary keys be mutable types like lists?

No, dictionary keys must be immutable types like strings, numbers, or tuples. Lists cannot be keys because they can change.

intermediate

What happens if you create a dictionary with duplicate keys?

The last value for the duplicate key will overwrite previous ones, so only the last key-value pair remains.

Which of these is the correct way to create a dictionary with keys 'a' and 'b' and values 1 and 2?

A {'a': 1, 'b': 2}

B ['a', 1, 'b', 2]

C ('a', 1, 'b', 2)

D [('a', 1), ('b', 2)]

Dictionaries use curly braces with key-value pairs separated by colons.

What is the output of type({}) in Python?

A <class 'tuple'>

B <class 'dict'>

C <class 'set'>

D <class 'list'>

Empty curly braces create an empty dictionary of type dict.

Which of these can be used as a dictionary key?

A [1, 2, 3]

B {'key': 'value'}

C [['a', 1]]

D (1, 2, 3)

Tuples are immutable and can be dictionary keys; lists and dictionaries cannot.

What will be the value of key 'x' after this dictionary is created? {'x': 1, 'x': 2}

A Error

B 1

C 2

D None

Duplicate keys keep the last assigned value, so 'x' will have value 2.

How do you create an empty dictionary using a function?

A dict()

B list()

C set()

D tuple()

The dict() function creates an empty dictionary.

Explain how to create a dictionary in Python and what rules apply to keys.

Think about how you store labeled information and what can be used as labels.

Describe what happens if you use duplicate keys when creating a dictionary.

Consider what happens if you write the same label twice on a list of notes.

Practice

1. Which of the following is the correct way to create an empty dictionary in Python?

easy

A. my_dict = ()

B. my_dict = []

C. my_dict = {}

D. my_dict = ''

Solution

1. Step 1: Understand dictionary syntax

   Dictionaries in Python are created using curly braces {}.

2. Step 2: Identify the empty dictionary

   An empty dictionary is represented as {}, not square brackets, parentheses, or quotes.

3. Final Answer:

   my_dict = {} -> Option C

4. Quick Check:

   Empty dictionary = {} [OK]

Hint: Use curly braces {} for dictionaries, brackets [] for lists [OK]

Common Mistakes:

• Using [] which creates a list, not a dictionary
• Using () which creates a tuple, not a dictionary
• Using '' which creates an empty string

2. Which of the following is the correct syntax to create a dictionary with keys 'a' and 'b' and values 1 and 2 respectively?

easy

A. my_dict = ['a':1, 'b':2]

B. my_dict = {'a'=1, 'b'=2}

C. my_dict = ('a':1, 'b':2)

D. my_dict = {'a':1, 'b':2}

Solution

1. Step 1: Check dictionary key-value pair syntax

   Dictionary pairs use colon : between key and value inside curly braces.

2. Step 2: Identify correct syntax

   my_dict = {'a':1, 'b':2} uses curly braces and colons correctly. Options A and C use wrong brackets, and D uses equals sign which is invalid.

3. Final Answer:

   my_dict = {'a':1, 'b':2} -> Option D

4. Quick Check:

   Dictionary pairs use : inside {} [OK]

Hint: Use colons : between keys and values inside {} [OK]

Common Mistakes:

• Using square brackets [] instead of curly braces {}
• Using parentheses () instead of curly braces {}
• Using equals sign = instead of colon :

3. What will be the output of the following code?

my_dict = {1: 'one', 2: 'two', 3: 'three'}
print(my_dict[2])

medium

A. 'two'

B. KeyError

C. 2

D. 'one'

Solution

1. Step 1: Understand dictionary key lookup

   Accessing my_dict[2] retrieves the value for key 2.

2. Step 2: Find value for key 2

   Key 2 maps to the string 'two' in the dictionary.

3. Final Answer:

   'two' -> Option A

4. Quick Check:

   my_dict[2] = 'two' [OK]

Hint: Dictionary[key] returns the value for that key [OK]

Common Mistakes:

• Confusing keys and values
• Expecting the key itself as output
• Mistaking KeyError when key exists

4. The following code throws an error. What is the problem?

my_dict = {1: 'one', 2: 'two', 3: 'three'}
print(my_dict[4])

medium

A. Key 4 does not exist in the dictionary

B. Syntax error in dictionary creation

C. Values must be integers, not strings

D. Dictionary keys must be strings

Solution

1. Step 1: Check dictionary keys

   The dictionary has keys 1, 2, and 3 only.

2. Step 2: Accessing a missing key

   Trying to access my_dict[4] causes a KeyError because key 4 is not present.

3. Final Answer:

   Key 4 does not exist in the dictionary -> Option A

4. Quick Check:

   Accessing missing key causes KeyError [OK]

Hint: Check if key exists before accessing dictionary [OK]

Common Mistakes:

• Assuming all keys exist
• Confusing syntax error with runtime error
• Thinking keys must be strings

5. You want to create a dictionary from two lists: keys = ['name', 'age', 'city'] and values = ['Alice', 30, 'NY']. Which code correctly creates this dictionary?

hard

A. my_dict = {keys: values}

B. my_dict = {keys[i]: values[i] for i in range(len(keys))}

C. my_dict = dict(keys, values)

D. my_dict = dict(zip(values, keys))

Solution

1. Step 1: Understand dictionary creation from two lists

   We need to pair each key with its corresponding value by index.

2. Step 2: Analyze options

   my_dict = {keys[i]: values[i] for i in range(len(keys))} uses dictionary comprehension with index to pair keys and values correctly. my_dict = dict(keys, values) is invalid syntax. my_dict = {keys: values} creates a dictionary with one key (the list) which is invalid. my_dict = dict(zip(values, keys)) reverses keys and values.

3. Final Answer:

   my_dict = {keys[i]: values[i] for i in range(len(keys))} -> Option B

4. Quick Check:

   Use dict comprehension with index to pair keys and values [OK]

Hint: Use dict comprehension with index or zip() to pair keys and values [OK]

Common Mistakes:

• Using dict() with two lists directly
• Swapping keys and values in zip()
• Using list as a key