Inverting a dictionary in Python - Mini Project: Build & Apply

Inverting a dictionary

📖 Scenario: Imagine you have a dictionary that stores the names of fruits as keys and their colors as values. You want to create a new dictionary that flips this information, so the colors become the keys and the fruits become the values.

🎯 Goal: Build a Python program that inverts a dictionary by swapping its keys and values.

📋 What You'll Learn

Create a dictionary with fruit names as keys and their colors as values

Create a variable to hold the inverted dictionary

Use a dictionary comprehension to invert the original dictionary

Print the inverted dictionary

💡 Why This Matters

🌍 Real World

Inverting dictionaries is useful when you want to look up information in reverse, like finding all fruits by their color instead of the other way around.

💼 Career

Understanding how to manipulate dictionaries and use comprehensions is important for data processing, software development, and many programming tasks.

1

Create the original dictionary

Create a dictionary called fruits with these exact entries: 'apple': 'red', 'banana': 'yellow', 'grape': 'purple', 'orange': 'orange', 'lemon': 'yellow'.

Python

# Create the dictionary called fruits with the given fruit-color pairs
# Your code here

Hint

Use curly braces {} to create the dictionary and separate each key-value pair with a comma.

2

Create an empty dictionary for the inverted data

Create an empty dictionary called inverted to hold the inverted fruit-color pairs.

Python

fruits = {'apple': 'red', 'banana': 'yellow', 'grape': 'purple', 'orange': 'orange', 'lemon': 'yellow'}
# Create an empty dictionary called inverted
# Your code here

Hint

Use empty curly braces {} to create an empty dictionary.

3

Invert the dictionary using dictionary comprehension

Use a dictionary comprehension to create the inverted dictionary by swapping keys and values from fruits. Use color and fruit as the variable names in the comprehension.

Python

fruits = {'apple': 'red', 'banana': 'yellow', 'grape': 'purple', 'orange': 'orange', 'lemon': 'yellow'}
inverted = {}
# Use dictionary comprehension to invert fruits into inverted
# Your code here

Hint

Use {key: value for key, value in dictionary.items()} format, swapping the key and value positions.

4

Print the inverted dictionary

Write a print statement to display the inverted dictionary.

Python

fruits = {'apple': 'red', 'banana': 'yellow', 'grape': 'purple', 'orange': 'orange', 'lemon': 'yellow'}
inverted = {color: fruit for fruit, color in fruits.items()}
# Print the inverted dictionary
# Your code here

Hint

Use print(inverted) to show the result.

Practice

1.

What does inverting a dictionary mean in Python?

easy

A. Sorting the dictionary by keys

B. Swapping keys and values so values become keys and keys become values

C. Removing duplicate keys from the dictionary

D. Changing all values to uppercase strings

Solution

1. Step 1: Understand dictionary structure

   A dictionary has keys and values paired together.

2. Step 2: Define inverting

   Inverting means swapping each key with its value, so keys become values and values become keys.

3. Final Answer:

   Swapping keys and values so values become keys and keys become values -> Option B

4. Quick Check:

   Inverting = swapping keys and values [OK]

Hint: Invert means swap keys and values in a dictionary [OK]

Common Mistakes:

• Thinking inverting sorts the dictionary
• Confusing inverting with removing duplicates
• Assuming values become uppercase strings

2.

Which of the following is the correct syntax to invert a dictionary d using dictionary comprehension?

easy

A. {k: v for k, v in d}

B. {k: v for v, k in d.items()}

C. {d[v]: d[k] for k, v in d.items()}

D. {v: k for k, v in d.items()}

Solution

1. Step 1: Recall dictionary comprehension syntax

   It uses {new_key: new_value for key, value in dict.items()}.

2. Step 2: Swap keys and values correctly

   To invert, new_key = value and new_value = key, so use {v: k for k, v in d.items()}.

3. Final Answer:

   {v: k for k, v in d.items()} -> Option D

4. Quick Check:

   Correct syntax = {v: k for k, v in d.items()} [OK]

Hint: Use {v: k for k, v in d.items()} to invert dictionary [OK]

Common Mistakes:

• Swapping variables incorrectly in comprehension
• Using d[v] or d[k] inside comprehension wrongly
• Forgetting to call .items() on dictionary

3.

What is the output of this code?

original = {'a': 1, 'b': 2, 'c': 3}
inverted = {v: k for k, v in original.items()}
print(inverted)

medium

A. {1: 'a', 2: 'b', 3: 'c'}

B. {'a': 1, 'b': 2, 'c': 3}

C. {'1': 'a', '2': 'b', '3': 'c'}

D. Error: unhashable type

Solution

1. Step 1: Understand original dictionary

   Keys are 'a', 'b', 'c' and values are 1, 2, 3.

2. Step 2: Invert dictionary using comprehension

   Swapping keys and values gives keys 1, 2, 3 and values 'a', 'b', 'c'.

3. Final Answer:

   {1: 'a', 2: 'b', 3: 'c'} -> Option A

4. Quick Check:

   Inverted dict = {1: 'a', 2: 'b', 3: 'c'} [OK]

Hint: Invert swaps keys and values exactly as pairs [OK]

Common Mistakes:

• Expecting original dictionary output
• Confusing string and integer keys
• Thinking inversion causes error here

4.

What is wrong with this code to invert a dictionary?

d = {'x': 10, 'y': 10}
inverted = {v: k for k, v in d.items()}
print(inverted)

medium

A. It will keep only one key for duplicate values

B. It will invert correctly with no issues

C. It will raise a TypeError

D. It will raise a KeyError

Solution

1. Step 1: Identify duplicate values in dictionary

   Both 'x' and 'y' have value 10, which is duplicated.

2. Step 2: Understand dictionary key uniqueness

   When inverting, keys must be unique, so only one key-value pair with key 10 remains.

3. Final Answer:

   It will keep only one key for duplicate values -> Option A

4. Quick Check:

   Duplicate values cause lost keys in inversion [OK]

Hint: Duplicate values become keys, only last key kept [OK]

Common Mistakes:

• Expecting all keys preserved after inversion
• Thinking it raises an error for duplicates
• Ignoring key uniqueness in dictionaries

5.

Given a dictionary with possible duplicate values, how can you invert it so each value maps to a list of keys that had that value?

original = {'a': 1, 'b': 2, 'c': 1}

Which code correctly inverts it to {1: ['a', 'c'], 2: ['b']}?

hard

A. inverted = {v: [k] for k, v in original.items()}

B. inverted = {v: k for k, v in original.items()}

C. inverted = {}
for k, v in original.items():
inverted.setdefault(v, []).append(k)

D. inverted = {v: k for v, k in original.items()}

Solution

1. Step 1: Understand problem with duplicates

   Simple inversion loses keys when values repeat, so we need lists to hold multiple keys.

2. Step 2: Use setdefault and append to collect keys

   Loop through items, for each value use setdefault to create list if missing, then append key.

3. Step 3: Check code correctness

   inverted = {} for k, v in original.items(): inverted.setdefault(v, []).append(k) uses this approach correctly, building lists of keys per value.

4. Final Answer:

   inverted = {} for k, v in original.items(): inverted.setdefault(v, []).append(k) -> Option C

5. Quick Check:

   Use setdefault + append to group keys by value [OK]

Hint: Use setdefault with append to group keys by value [OK]

Common Mistakes:

• Using simple comprehension losing duplicate keys
• Swapping variables incorrectly in comprehension
• Expecting one-to-one inversion with duplicates