groupby

groupby() groups consecutive elements with the same key, making it useful for processing sorted data. Unlike SQL-style GROUP BY or pandas.groupby, this function only groups elements that are adjacent in the input sequence. To group all matching elements together, sort the input by the key first. This iterator-based approach works well for streaming data where items arrive in order.

Basic Grouping

Without a key function, groupby() groups consecutive elements that are equal. Note that the trailing 1 in the example forms its own group because it is separated from the earlier 1s by other values.

from itertools import groupby

data = [1, 1, 1, 2, 2, 3, 3, 3, 3, 1]
grouped = groupby(data)

for key, group in grouped:
    print(f"{key}: {list(group)}")

1: [1, 1, 1]
2: [2, 2]
3: [3, 3, 3, 3]
1: [1]

The trailing 1 appears as a separate group because groupby() only groups consecutive elements. To group all 1s together, sort the data first.

Grouping with Key Function

A key function maps each element to a grouping value. groupby() starts a new group whenever the key value changes, so the input should be sorted by the same key for exhaustive grouping.

from itertools import groupby

words = ['apple', 'apricot', 'banana', 'blueberry', 'cherry']
# Group by first letter
grouped = groupby(words, key=lambda x: x[0])

for letter, items in grouped:
    print(f"{letter}: {list(items)}")

a: ['apple', 'apricot']
b: ['banana', 'blueberry']
c: ['cherry']