groupby
- Python course
- Intermediate
- Lesson 98/116
- 5 min
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.
Practice 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))).
Starter code
# Count groups with groupbySample solution
# Count groups with groupby
from itertools import groupby
for k, g in groupby([1, 1, 2, 2, 3]):
print(k, len(list(g)))Expected output
1 2 2 2 3 1
Questions and answers
Review the lesson questions together with their answers. To test yourself, solve the lesson interactively.
1 What does groupby expect the data to be?
- Random order
- Reverse sorted
- Sorted by key Correct answer
- Unsorted
Explanation: groupby expects the data to be sorted by key.
2 groupby loop
from itertools import groupby
for k, g in ____([1, 1, 2, 2, 3]):
print(k, len(list(g)))from itertools import groupby
for k, g in groupby([1, 1, 2, 2, 3]):
print(k, len(list(g)))Explanation: groupby creates consecutive groups.
3 Match the groupby parts
| Term | Meaning |
|---|---|
groupby |
Grouping function |
key |
Key function |
(k, g) |
Key and group |
list(g) |
Convert to list |