groupby

Learn grouping with groupby.

groupby

groupby(iterable, key=fn) groups consecutive items with the same key in an iterable. Each group is returned as a (key, group_iterator) pair.

Important: groupby only groups consecutive same keys. The data must be sorted by key beforehand; otherwise the same key can appear in multiple groups. If key is not given, the items themselves are the keys.

The group iterator exhausts

The group g returned from groupby is an iterator. Once you walk it, it exhausts; walking again returns nothing. So a common practice is to convert the group to a list immediately with list(g).

Example: for k, g in groupby([1, 1, 2]):\n print(k, list(g)). This code prints the key and the items as a list for each group. If you want to use the group iterator more than once, convert it to a list and store it.

Exercise Count groups with groupby

Start with from itertools import groupby. Write the loop for k, g in groupby([1, 1, 2, 2, 3]):. In the body print each group's key and count with print(k, len(list(g))).

Show example solution

# Count groups with groupby
from itertools import groupby

for k, g in groupby([1, 1, 2, 2, 3]):
    print(k, len(list(g)))

Test yourself

What does groupby expect the data to be?
- Random order
- Reverse sorted
- Sorted by key
- Unsorted
Show answer

Sorted by key — groupby expects the data to be sorted by key.

groupby loop

from itertools import groupby

for k, g in ____([1, 1, 2, 2, 3]):
    print(k, len(list(g)))

Show answer

from itertools import groupby

for k, g in groupby([1, 1, 2, 2, 3]):
    print(k, len(list(g)))

Match the groupby parts

groupby

Grouping function
key

Key function
(k, g)

Key and group
list(g)

Convert to list

Next lesson: Summary - Itertools

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