datetime.fromisoformat() accepts invalid ISO 8601 timestamps

[Paebbels]

Bug report

Bug description:

Since Python 3.11, datetime.fromisoformat(...) accepts invalid timestamps. The same code was tested in many Python versions via GitHub Actions:

Wrong format: 2024-01-17T15:21:00-0800
Fixed format: 2024-01-17T15:21:00-08:00

Python code to check:

datetime.fromisoformat("2024-01-17T15:21:00-0800")

ISO 8601 defines 2 formats:

• basic format (in short: without separators) and
• extended format (with separators).

When checking the EBNF rules, all parts of the timestamp must be with or without separators. A mix of formats is not allowed.

I haven't checked for other basic and extended format mixes.
My CI checked many Python versions and platforms. Here a failed CI pipeline, shows a correct implementation. A successful pipeline denotes a buggy Python version.

Other resources:

• datetime.strptime: Support for parsing offsets with a colon #75981
• datetime.fromisoformat(): Omitted colon in timezone suffix raises ValueError #86537
• datetime.fromisoformat incorrectly accepts and parses strings without date-time separator #107779
  • gh-107779: Check for the separator between date and time in datetime.datetime.fromisoformat #107791
• parser.isoparse() accepts invalid ISO 8601 timestamps dateutil/dateutil#1341
• ISO 8601 timestamps - basic/extended format mixing adlnet/xAPI-Spec#541
• ISO8601 - Mixed Basic and Extended format bitwalker/timex#98
• ISO 8601 - https://web.archive.org/web/20171019211402/https://www.loc.gov/standards/datetime/ISO_DIS%208601-1.pdf

CPython versions tested on:

3.8, 3.9, 3.10, 3.11, 3.12, 3.13

Operating systems tested on:

Linux, macOS, Windows

[skoehler]

I'd like to extend the list of accepted invalid timestamps:

2024-01-17T15:21:00-0800
2024-01-17T152100-08:00
20240117T15:21:00-08:00

2024-01-17T152100-0800
20240117T15:21:00-0800
20240117T152100-08:00

All these should be rejected by fromisoformat().
I can report however, that removing just a single dash or a single colon from the date or time is properly rejected.

[TooPercentMilk]

I'm a beginner contributor and I'd like to work on this issue. How can I move forward?

[maxcyd]

It is worth noting, the test_datetime defined within datetimetester.py test assumes inconsistent typing is acceptable. For example, '20250102T03:04:05,6' is considered an acceptable string to the datetime.isoformat() function.