Inverting a dictionary in Python - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does it mean to invert a dictionary?

Inverting a dictionary means swapping its keys and values. The original keys become values, and the original values become keys.

beginner

How can you invert a dictionary in Python using a dictionary comprehension?

You can invert a dictionary by creating a new dictionary where each key-value pair is swapped:

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

intermediate

What problem can occur if the original dictionary has duplicate values when inverting?

If the original dictionary has duplicate values, inverting it will lose data because keys in a dictionary must be unique. Only the last key for a duplicated value will be kept.

beginner

Show a Python code example that inverts a dictionary and explain the output.

original = {'a': 1, 'b': 2, 'c': 3}

Code:

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

Output:

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

Explanation: Each key-value pair is swapped, so 1 becomes a key with value 'a', and so on.

intermediate

How can you invert a dictionary that has duplicate values without losing data?

You can invert it by making the values keys that map to a list of original keys:

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

What happens if you invert a dictionary with duplicate values using a simple comprehension?

A It will raise an error

B Some keys will be lost because dictionary keys must be unique

C The dictionary will keep all keys and values as is

D The dictionary will become empty

When inverting, duplicate values become keys, but keys must be unique, so only the last key-value pair is kept.

Which Python method helps to iterate over key-value pairs in a dictionary?

A items()

B keys()

C values()

D get()

The items() method returns key-value pairs, which is useful for inverting dictionaries.

What is the output of this code?

original = {'x': 10, 'y': 20}
inverted = {v: k for k, v in original.items()}
print(inverted)

A {'x': 10, 'y': 20}

B {'10': 'x', '20': 'y'}

C {10: 'x', 20: 'y'}

D Error

The dictionary is inverted by swapping keys and values.

How can you invert a dictionary and keep all original keys if values are duplicated?

A Use keys() method only

B Use a simple comprehension

C Ignore duplicates

D Map each value to a list of keys

Mapping values to lists of keys preserves all keys even if values repeat.

What type of Python object is created when you invert a dictionary?

A Another dictionary

B A list

C A set

D A tuple

Inverting a dictionary produces a new dictionary with keys and values swapped.

Explain how to invert a dictionary in Python and what issues might arise with duplicate values.

Think about how keys must be unique in dictionaries.

Write a Python code snippet to invert a dictionary that may have duplicate values, preserving all original keys.

Use a dictionary where each key maps to a list.

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