Values shared by multiple keys
Input
invert_mapping({'a': 1, 'b': 1, 'c': 2})
Output
{1: ['a', 'b'], 2: ['c']}
'a' and 'b' both mapped to 1, so 1 maps to the list ['a', 'b'].
Full lesson preview
Invert a Dictionary with Duplicate Values
Create an inverted mapping where each original value maps to the list of keys that referenced it.
Python practice16 minList & Dictionary PatternsIntermediateLast updated March 20, 2026
Problem statement
Implement invert_mapping(d) which returns a new dictionary mapping each value found in d to a list of keys that originally pointed to that value. Always produce lists (even if a value had a single key). For deterministic output, the lists of keys must be sorted using the default Python ordering when possible.
Task
Write a function to invert a dictionary so that values become keys and values are lists of original keys (sorted for determinism).
Examples
Input format
A dict d with hashable keys and hashable values.
Output format
A dict mapping each original value to a sorted list of keys that had that value.
Constraints
Lists of keys should be sorted for determinism; do not attempt to coerce or merge different types beyond sorting where possible.
Samples
Sample 1
Input
invert_mapping({'a': 1, 'b': 2})
Output
{1: ['a'], 2: ['b']}
Simple inversion where each value is unique.