Add Easy Key-Value Aggregates With defaultdict as a Python TIL

python/easy-key-value-aggregates-with-defaultdict.md

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+# Easy Key-Value Aggregates With defaultdict
+
+The `collections` module has the `defaultdict` object that can be used to
+aggregate values tied to a key. What sets this apart from simply using a `dict`
+is that we get the base value for free. So if our aggregate value is a list,
+then we get `[]` by default for each new key. In the same way, we'd get `0` if
+it was constructed with `int`.
+
+Here is the counter example from [Keep A Tally With
+collections.Counter](keep-a-tally-with-collections-counter.md)
+
+```python
+from collections import defaultdict
+
+def get_pair_counts(token_ids: list[int]) -> Counter:
+    """Count how often each adjacent pair appears"""
+    counts = defaultdict(int)
+    for i in range(len(token_ids) - 1):
+        pair = (token_ids[i], token_ids[i + 1])
+        counts[pair] += 1
+    return counts
+```
+
+We never have to initially set a key to `0`. If the key is not yet present, then
+`int()` (the zero-value constructor) is used as the `__missing__` value.
+
+We can do the same with `list`:
+
+```python
+>>> import collections
+>>> stuff = collections.defaultdict(list)
+>>> stuff['alpha'].append(1)
+>>> stuff['alpha']
+[1]
+>>> stuff['beta']
+[]
+```
+
+In the same way, this uses `list()` as the `__missing__` value to start of each
+key with an `[]`.
+
+I find this so handy because in other languages I've typically had to do
+something more like this:
+
+```python
+words_by_length = {}
+for item in items:
+    if len(item) not in words_by_length:
+        words_by_length[len(item)] = []
+    words_by_length[len(item)].append(item)
+```
+
+This is much clunkier.