group by does not sort data before grouping

[balakhaniyan]

Bug report

Bug description:

itertools.groupby(dynamic_properties, lambda d: d.table_id)

When data is not sorted base on key, group by does not work correctly and it needs to be sorted...
Isn't it a bug?

CPython versions tested on:

3.13

Operating systems tested on:

Windows

[picnixz]

I don't think groupby is meant to sort data (at least the docs don't guarantee it). For instance,
"".join([k for k, g in groupby('AAAABBBCCDAABBB')]) returns ABCDAB.

More precisely, the docs mention (emphasis mine):

The operation of groupby() is similar to the uniq filter in Unix. It generates a break or new group every time the value of the key function changes (which is why it is usually necessary to have sorted the data using the same key function). That behavior differs from SQL’s GROUP BY which aggregates common elements regardless of their input order

This is because sorting data before would exhaust iterators but also add an additional cost that may be superfluous in some situations.