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

Concept Flow - Inverting a dictionary

Start with original dictionary

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For each key-value pair

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Swap key and value

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Add swapped pair to new dictionary

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Repeat for all pairs

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Return inverted dictionary

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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.