Cannot parse 2026-W36-1 specifically

[AetherUnbound]

• I am on the latest Pendulum version.
• I have searched the issues of this repo and believe that this is not a duplicate.

• OS version and name: OSX 15.6.1 (also seeing this on a Debian docker image)
• Pendulum version: 3.0.0

Issue

Pendulum appears unable to parse 2026W36, or specifically 2026-W36-1 of the ISO 8601 spec. Here's some examples:

>>> import pendulum
>>> from pendulum import DateTime, Timezone
>>> pendulum.parse("2026W36", strict=False)
Traceback (most recent call last):
  File "/Users/aether/git/airflow/etl/.venv/lib/python3.12/site-packages/pendulum/parsing/__init__.py", line 128, in _parse
    dt = parser.parse(
         ^^^^^^^^^^^^^
  File "/Users/aether/git/airflow/etl/.venv/lib/python3.12/site-packages/dateutil/parser/_parser.py", line 1368, in parse
    return DEFAULTPARSER.parse(timestr, **kwargs)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/aether/git/airflow/etl/.venv/lib/python3.12/site-packages/dateutil/parser/_parser.py", line 643, in parse
    raise ParserError("Unknown string format: %s", timestr)
dateutil.parser._parser.ParserError: Unknown string format: 2026W36

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/Users/aether/git/airflow/etl/.venv/lib/python3.12/site-packages/pendulum/parser.py", line 30, in parse
    return _parse(text, **options)
           ^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/aether/git/airflow/etl/.venv/lib/python3.12/site-packages/pendulum/parser.py", line 43, in _parse
    parsed = base_parse(text, **options)
             ^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/aether/git/airflow/etl/.venv/lib/python3.12/site-packages/pendulum/parsing/__init__.py", line 78, in parse
    return _normalize(_parse(text, **_options), **_options)
                      ^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/aether/git/airflow/etl/.venv/lib/python3.12/site-packages/pendulum/parsing/__init__.py", line 132, in _parse
    raise ParserError(f"Invalid date string: {text}")
pendulum.parsing.exceptions.ParserError: Invalid date string: 2026W36
# Previous year works just fine
>>> pendulum.parse("2025W36", strict=False)
DateTime(2025, 9, 1, 0, 0, 0, tzinfo=Timezone('UTC'))
# Other dates in same year work
>>> pendulum.parse("2026W01", strict=False)
DateTime(2025, 12, 29, 0, 0, 0, tzinfo=Timezone('UTC'))
>>> pendulum.parse("2026W02", strict=False)
DateTime(2026, 1, 5, 0, 0, 0, tzinfo=Timezone('UTC'))
# Week prior and week after work
>>> pendulum.parse("2026W35", strict=False)
DateTime(2026, 8, 24, 0, 0, 0, tzinfo=Timezone('UTC'))
>>> pendulum.parse("2026W37", strict=False)
DateTime(2026, 9, 7, 0, 0, 0, tzinfo=Timezone('UTC'))
# Using the day declaration fails multiple ways
>>> pendulum.parse("2025W36-1", strict=False)
Traceback (most recent call last):
  File "/Users/aether/git/airflow/etl/.venv/lib/python3.12/site-packages/pendulum/parsing/__init__.py", line 128, in _parse
    dt = parser.parse(
         ^^^^^^^^^^^^^
  File "/Users/aether/git/airflow/etl/.venv/lib/python3.12/site-packages/dateutil/parser/_parser.py", line 1368, in parse
    return DEFAULTPARSER.parse(timestr, **kwargs)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/aether/git/airflow/etl/.venv/lib/python3.12/site-packages/dateutil/parser/_parser.py", line 643, in parse
    raise ParserError("Unknown string format: %s", timestr)
dateutil.parser._parser.ParserError: Unknown string format: 2025W36-1

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/Users/aether/git/airflow/etl/.venv/lib/python3.12/site-packages/pendulum/parser.py", line 30, in parse
    return _parse(text, **options)
           ^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/aether/git/airflow/etl/.venv/lib/python3.12/site-packages/pendulum/parser.py", line 43, in _parse
    parsed = base_parse(text, **options)
             ^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/aether/git/airflow/etl/.venv/lib/python3.12/site-packages/pendulum/parsing/__init__.py", line 78, in parse
    return _normalize(_parse(text, **_options), **_options)
                      ^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/aether/git/airflow/etl/.venv/lib/python3.12/site-packages/pendulum/parsing/__init__.py", line 132, in _parse
    raise ParserError(f"Invalid date string: {text}")
pendulum.parsing.exceptions.ParserError: Invalid date string: 2025W36-1
>>> pendulum.parse("2026-W36-1", strict=False)
Traceback (most recent call last):
  File "/Users/aether/git/airflow/etl/.venv/lib/python3.12/site-packages/pendulum/parsing/__init__.py", line 128, in _parse
    dt = parser.parse(
         ^^^^^^^^^^^^^
  File "/Users/aether/git/airflow/etl/.venv/lib/python3.12/site-packages/dateutil/parser/_parser.py", line 1368, in parse
    return DEFAULTPARSER.parse(timestr, **kwargs)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/aether/git/airflow/etl/.venv/lib/python3.12/site-packages/dateutil/parser/_parser.py", line 643, in parse
    raise ParserError("Unknown string format: %s", timestr)
dateutil.parser._parser.ParserError: Unknown string format: 2026-W36-1

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/Users/aether/git/airflow/etl/.venv/lib/python3.12/site-packages/pendulum/parser.py", line 30, in parse
    return _parse(text, **options)
           ^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/aether/git/airflow/etl/.venv/lib/python3.12/site-packages/pendulum/parser.py", line 43, in _parse
    parsed = base_parse(text, **options)
             ^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/aether/git/airflow/etl/.venv/lib/python3.12/site-packages/pendulum/parsing/__init__.py", line 78, in parse
    return _normalize(_parse(text, **_options), **_options)
                      ^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/aether/git/airflow/etl/.venv/lib/python3.12/site-packages/pendulum/parsing/__init__.py", line 132, in _parse
    raise ParserError(f"Invalid date string: {text}")
pendulum.parsing.exceptions.ParserError: Invalid date string: 2026-W36-1
>>> pendulum.parse("2026-W36", strict=False)
Traceback (most recent call last):
  File "/Users/aether/git/airflow/etl/.venv/lib/python3.12/site-packages/pendulum/parsing/__init__.py", line 128, in _parse
    dt = parser.parse(
         ^^^^^^^^^^^^^
  File "/Users/aether/git/airflow/etl/.venv/lib/python3.12/site-packages/dateutil/parser/_parser.py", line 1368, in parse
    return DEFAULTPARSER.parse(timestr, **kwargs)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/aether/git/airflow/etl/.venv/lib/python3.12/site-packages/dateutil/parser/_parser.py", line 643, in parse
    raise ParserError("Unknown string format: %s", timestr)
dateutil.parser._parser.ParserError: Unknown string format: 2026-W36

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/Users/aether/git/airflow/etl/.venv/lib/python3.12/site-packages/pendulum/parser.py", line 30, in parse
    return _parse(text, **options)
           ^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/aether/git/airflow/etl/.venv/lib/python3.12/site-packages/pendulum/parser.py", line 43, in _parse
    parsed = base_parse(text, **options)
             ^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/aether/git/airflow/etl/.venv/lib/python3.12/site-packages/pendulum/parsing/__init__.py", line 78, in parse
    return _normalize(_parse(text, **_options), **_options)
                      ^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/aether/git/airflow/etl/.venv/lib/python3.12/site-packages/pendulum/parsing/__init__.py", line 132, in _parse
    raise ParserError(f"Invalid date string: {text}")
pendulum.parsing.exceptions.ParserError: Invalid date string: 2026-W36
# Confirmation that day declaration works in general
>>> pendulum.parse("2025-W36-1", strict=False)
DateTime(2025, 9, 1, 0, 0, 0, tzinfo=Timezone('UTC'))
>>> pendulum.parse("2026-W01-1", strict=False)
DateTime(2025, 12, 29, 0, 0, 0, tzinfo=Timezone('UTC'))
# Week prior works
>>> pendulum.parse("2026-W37-1", strict=False)
DateTime(2026, 9, 7, 0, 0, 0, tzinfo=Timezone('UTC'))
# Day after and day before work
>>> pendulum.parse("2026-W36-2", strict=False)
DateTime(2026, 9, 1, 0, 0, 0, tzinfo=Timezone('UTC'))
>>> pendulum.parse("2026-W35-7", strict=False)
DateTime(2026, 8, 30, 0, 0, 0, tzinfo=Timezone('UTC'))
# Date can be constructed on its own just fine
>>> DateTime(2026, 8, 31, 0, 0, 0, tzinfo=Timezone('UTC'))
DateTime(2026, 8, 31, 0, 0, 0, tzinfo=Timezone('UTC'))

I'm really not sure what's going on here! This should ostensibly be working and 2026-08-31 should be a valid date 😅 Any pointers here would be wonderful!

Thank you!

[AetherUnbound]

Apparently 2026W49 causes the same issue 🥲

>>> pendulum.parse("2026W49", strict=False)
Traceback (most recent call last):
  File "/Users/aether/git/airflow/etl/.venv/lib/python3.12/site-packages/pendulum/parsing/__init__.py", line 128, in _parse
    dt = parser.parse(
         ^^^^^^^^^^^^^
  File "/Users/aether/git/airflow/etl/.venv/lib/python3.12/site-packages/dateutil/parser/_parser.py", line 1368, in parse
    return DEFAULTPARSER.parse(timestr, **kwargs)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/aether/git/airflow/etl/.venv/lib/python3.12/site-packages/dateutil/parser/_parser.py", line 643, in parse
    raise ParserError("Unknown string format: %s", timestr)
dateutil.parser._parser.ParserError: Unknown string format: 2026W49

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/Users/aether/git/airflow/etl/.venv/lib/python3.12/site-packages/pendulum/parser.py", line 30, in parse
    return _parse(text, **options)
           ^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/aether/git/airflow/etl/.venv/lib/python3.12/site-packages/pendulum/parser.py", line 43, in _parse
    parsed = base_parse(text, **options)
             ^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/aether/git/airflow/etl/.venv/lib/python3.12/site-packages/pendulum/parsing/__init__.py", line 78, in parse
    return _normalize(_parse(text, **_options), **_options)
                      ^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/aether/git/airflow/etl/.venv/lib/python3.12/site-packages/pendulum/parsing/__init__.py", line 132, in _parse
    raise ParserError(f"Invalid date string: {text}")
pendulum.parsing.exceptions.ParserError: Invalid date string: 2026W49

FWIW the common denominator appears to be that the default day for the week falls on the last day of the month (W36 fell on 2026-08-31):

>>> pendulum.parse("2026-W49-2", strict=False)
DateTime(2026, 12, 1, 0, 0, 0, tzinfo=Timezone('UTC'))
>>> pendulum.parse("2026-W48-7", strict=False)
DateTime(2026, 11, 29, 0, 0, 0, tzinfo=Timezone('UTC'))
# Presumably 2026-W49-1 would be 2026-11-30

[AetherUnbound]

This seems to be a valid workaround (use previous week's day 7, then add 1 day):

>>> pendulum.parse("2026-W35-7", strict=False) + pendulum.duration(days=1)
DateTime(2026, 8, 31, 0, 0, 0, tzinfo=Timezone('UTC'))
>>> pendulum.parse("2026-W48-7", strict=False) + pendulum.duration(days=1)
DateTime(2026, 11, 30, 0, 0, 0, tzinfo=Timezone('UTC'))

Here's the full logic I've got working for this, in case this is useful to anyone else:

try:
    dt = pendulum.parse(f"{year}{week}".upper(), strict=False).to_date_string()
except pendulum.parsing.exceptions.ParserError:
    week = int(week.replace("W", ""))
    year = int(year)
    if week == 1:
        year -= 1
        week = 52
    else:
        week -= 1

    parsed = pendulum.parse(f"{year}-W{week:02}-7".upper(), strict=False)
    dt = (parsed + pendulum.duration(days=1)).to_date_string()

[AetherUnbound]

This looks like it's actually a difference between the rust implementation and the Python one!

>>> from pendulum.parsing.iso8601 import parse_iso8601 as p_python
>>> from pendulum._pendulum import parse_iso8601 as p_rust
>>> p_python("2026W36")
datetime.date(2026, 8, 31)
>>> p_rust("2026W36")
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
ValueError: day is out of range for month