Summary

pendulum.parse() rejects valid ISO 8601 week dates whose year is below 1000, even though the same years parse fine as ordinary calendar dates, and datetime.fromisoformat() accepts them:

>>> import pendulum
>>> pendulum.parse("2016-W34-6")
2016-08-27 00:00:00+00:00
>>> pendulum.parse("1000-W34-6")
1000-08-23 00:00:00+00:00
>>> pendulum.parse("0999-W34-6")
pendulum.parsing.exceptions.ParserError: Unable to parse string [0999-W34-6]
>>> pendulum.parse("0416-08-21")          # same year, calendar form: fine
0416-08-21 00:00:00+00:00
>>> pendulum.parse("0416-W34-6")
ParserError: Unable to parse string [0416-W34-6]
>>> pendulum.parse("0416W346")            # basic format: also broken
ParserError: Unable to parse string [0416W346]

All of 0999-W34-6, 0416-W34-6, 0416W346, 0416-W34, 0004-W10-5 raise ParserError while datetime.fromisoformat parses each of them.

Root cause

src/pendulum/parsing/iso8601.py, _get_iso_8601_week (lines ~448-452):

fmt = "%Y-%j"
string = f"{year}-{ordinal}"

dt = datetime.datetime.strptime(string, fmt)

The year is interpolated without zero-padding, so for year 416 the string is "416-240". strptime's %Y matches exactly four digits, so the lookup fails with ValueError: time data '416-240' does not match format '%Y-%j', which _parse then masks into the generic ParserError("Unable to parse string [...]").

The boundary is exact: years ≥ 1000 work, years ≤ 999 fail.

Expected behavior

Zero-pad the year (f"{year:04d}"), matching what the %Y directive requires. 0416-W34-6 then parses to 0416-08-27, same as datetime.fromisoformat.

Environment

• pendulum 3.1.0 (current master, e06dace), Python 3.13, Linux.