What is the `itertools.groupby` function?

Question

Accepted Answer

**groupby** — groups consecutive elements by a key function.

```python
import itertools

# Group sorted data
data = [
    {"type": "A", "value": 1},
    {"type": "B", "value": 2},
    {"type": "A", "value": 3},
    {"type": "B", "value": 4},
]

# Must sort by key first!
data.sort(key=lambda x: x["type"])

for key, group in itertools.groupby(data, key=lambda x: x["type"]):
    print(f"{key}: {[item["value"] for item in group]}")
# A: [1, 3]
# B: [2, 4]

# Simple grouping
numbers = [1, 1, 2, 2, 2, 3, 3]
for key, group in itertools.groupby(numbers):
    print(f"{key}: {list(group)}")
# 1: [1, 1]
# 2: [2, 2, 2]
# 3: [3, 3]
```

**Key point:** Data MUST be sorted by the grouping key first, or groups will be split.

Python: What is the `itertools.groupby` function?