Basic merge
Input
merge_dicts_by_key([{'id':1,'a':10},{'id':1,'b':20}], 'id')
Output
{1: {'a': 10, 'b': 20}}
Two records share id=1. Fields 'a' and 'b' are merged into the single result for key 1.
Full lesson preview
Merge a list of dictionaries by key
Group a list of dictionaries by a specified key and merge their fields into combined entries. Conflicting values become lists of unique values in order encountered.
Python practice13 minList & Dictionary PatternsIntermediateLast updated March 20, 2026
Problem statement
Given a list of dictionaries and a key name, group the dictionaries by the value at that key. For each group produce a single merged dictionary (excluding the grouping key itself) where:
- If a field has the same value across all items in the group, keep it as a scalar.
- If a field has different values across the group, collect them into a list of unique values in the order they were encountered.
- Skip any input dictionaries that do not contain the grouping key.
Return a dictionary that maps each distinct key value to its merged dictionary.
This is useful when combining rows of data with a shared identifier and you want to aggregate differing attributes without losing information.
Task
Implement a function that groups dictionaries by a given key and merges other fields, producing a mapping from key value to merged dictionary.
Examples
Input format
A list of dictionaries and a string key name: (items, key)
Output format
A dictionary mapping each key value to a merged dictionary of the remaining fields.
Constraints
- Items that are not dicts or that don't contain the key should be ignored.
- Preserve the order of encountered values when creating lists.
- Use only standard Python; aim for O(n * m) where n is number of items and m average fields per item.
Samples
Sample 1
Input
merge_dicts_by_key([{'id':'x','v':1},{'id':'x','v':2},{'id':'y','v':3}], 'id')
Output
{'x': {'v': [1, 2]}, 'y': {'v': 3}}
Key 'x' had two different v values so they are collected into a list; 'y' had a single value.