A grouped dictionary differently

The two approaches you’ve described handle creating and populating a grouped dictionary differently. Let’s break them down:

1. grouped = defaultdict(list)

• defaultdict is a subclass of Python’s dict that provides a default value for a key if it doesn’t exist.
• Here, list is passed to defaultdictThis means that any new key will automatically initialize with an empty list.

Example with grouped[sorted_value].append(key):

from collections import defaultdict

grouped = defaultdict(list)  # Default value for new keys is an empty list.
data = [("cat", "tac"), ("bat", "tab"), ("rat", "tar")]

for key, value in data:
    sorted_value = "".join(sorted(value))
    grouped[sorted_value].append(key)  # Appends the key to the list of values.

print(grouped)
# Output: {'act': ['cat'], 'abt': ['bat'], 'art': ['rat']}

Use Case: This is great when you want multiple keys to map to the same “group” (e.g., anagrams) and store all these keys in a list.

2. grouped = {}

• A plain dictionary is used, and no default values are provided. You must check if a key exists before appending.

Example with grouped[sorted_value] = key:

grouped = {}
data = [("cat", "tac"), ("bat", "tab"), ("rat", "tar")]

for key, value in data:
    sorted_value = "".join(sorted(value))
    grouped[sorted_value] = key  # Overwrites any previous value.

print(grouped)
# Output: {'act': 'cat', 'abt': 'bat', 'art': 'rat'}

• Behavior:
  • In this case, each sorted_value maps to only one key. If sorted_value is repeated, the last key will overwrite the previous value.
  • This approach is suitable when each key-value pair in the dictionary must be unique.

Example with grouped[sorted_value].append(key) (without defaultdict):

grouped = {}

data = [("cat", "tac"), ("bat", "tab"), ("rat", "tar")]

for key, value in data:
    sorted_value = "".join(sorted(value))
    if sorted_value not in grouped:
        grouped[sorted_value] = []  # Initialize an empty list for new keys.
    grouped[sorted_value].append(key)

print(grouped)
# Output: {'act': ['cat'], 'abt': ['bat'], 'art': ['rat']}

Behaviour: Without defaultdict, you must explicitly check if the key exists and initialize it. This is less concise compared to using defaultdict.

Conclusion:

• Use defaultdict(list) when grouping multiple values (e.g., grouping items by category, finding anagrams).
• Use a plain dictionary (grouped = {}) when each key maps to a single value, or when you need fine-grained control over how values are assigned.