accumulate values by key
Input
[('a', 1), ('b', 2), ('a', 3)]
Output
{'a': [1, 3], 'b': [2]}
Values for 'a' are collected in the list [1, 3] in the order encountered.
Full lesson preview
Convert pairs into a dictionary of lists
Turn a list of (key, value) pairs into a dictionary mapping keys to lists of their values.
Python practice17 minList & Dictionary PatternsIntermediateLast updated March 20, 2026
Problem statement
Given a list of 2-tuples (key, value) pairs, implement pairs_to_dict(pairs) that returns a dictionary mapping each key to a list of values that appeared paired with that key, preserving input order for values. Keys can be of any hashable type. The returned dictionary must be deterministic: insert keys ordered by str(key).
Task
Implement pairs_to_dict(pairs) to collect values for each key into lists, preserving the order of values for each key. Return a dict with keys inserted in order sorted by str(key) for deterministic output.
Examples
Input format
A single parameter: pairs (list of 2-tuples).
Output format
A dict mapping each key to a list of values.
Constraints
- Preserve the order of values for each key. - Use the original key objects in the returned dict. - For deterministic output, insert keys ordered by str(key).
Samples
Sample 1
Input
[(1, 'one'), (2, 'two'), (1, 'uno')]
Output
{1: ['one', 'uno'], 2: ['two']}
Integer keys are preserved; lists keep insertion order.