Inverting a dictionary in Python - Step-by-Step Execution

Concept Flow - Inverting a dictionary

Start with original dictionary

↓

For each key-value pair

↓

Swap key and value

↓

Add swapped pair to new dictionary

↓

Repeat for all pairs

↓

Return inverted dictionary

↓

End

We take each key and value from the original dictionary, swap them, and put them into a new dictionary until all pairs are processed.

Execution Sample

Python

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

This code creates a new dictionary by swapping keys and values from the original dictionary.

Execution Table

Step	Current key	Current value	Action	Inverted dictionary state
1	'a'	1	Add 1:'a' to inverted	{1: 'a'}
2	'b'	2	Add 2:'b' to inverted	{1: 'a', 2: 'b'}
3	'c'	3	Add 3:'c' to inverted	{1: 'a', 2: 'b', 3: 'c'}
4	No more items	-	Finish loop	{1: 'a', 2: 'b', 3: 'c'}

💡 All key-value pairs processed, loop ends.

Variable Tracker

Variable	Start	After 1	After 2	After 3	Final
original	{'a': 1, 'b': 2, 'c': 3}	{'a': 1, 'b': 2, 'c': 3}	{'a': 1, 'b': 2, 'c': 3}	{'a': 1, 'b': 2, 'c': 3}	{'a': 1, 'b': 2, 'c': 3}
inverted	{}	{1: 'a'}	{1: 'a', 2: 'b'}	{1: 'a', 2: 'b', 3: 'c'}	{1: 'a', 2: 'b', 3: 'c'}

Key Moments - 2 Insights

Why do the keys and values swap places in the new dictionary?

Because in the comprehension, we write {v: k} which means the original value becomes the new key, and the original key becomes the new value, as shown in execution_table steps 1-3.

What happens if two original keys have the same value?

The last key processed will overwrite the previous one in the inverted dictionary because dictionary keys must be unique. This is not shown here but is important to know.

Visual Quiz - 3 Questions

Look at the execution table, what is the inverted dictionary after step 2?

A {2: 'b'}

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

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

D {}

💡 Hint

Check the 'Inverted dictionary state' column at step 2 in the execution_table.

At which step does the loop finish processing all items?

A Step 3

B Step 2

C Step 4

D Step 1

💡 Hint

Look for the step where 'No more items' is the current key in the execution_table.

If the original dictionary had two keys with value 1, what would happen in the inverted dictionary?

A Only one key would be stored for key 1, the last one processed

B Both keys would appear as values for key 1

C An error would occur

D The inverted dictionary would be empty

💡 Hint

Recall that dictionary keys must be unique; see key_moments about overwriting.

Concept Snapshot

Inverting a dictionary swaps keys and values.
Use a dictionary comprehension: {v: k for k, v in original.items()}.
Each original value becomes a new key.
Duplicates in values cause overwriting.
Result is a new dictionary with swapped pairs.

Full Transcript

We start with an original dictionary. For each key and value, we swap them and add to a new dictionary. This process repeats until all pairs are processed. The final result is the inverted dictionary where original values are keys and original keys are values. If original values repeat, the last key overwrites previous ones in the inverted dictionary.

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