ISO date errors in _strptime are jumbled

[GPHemsley]

BPO	36959
Nosy	@pganssle, @GPHemsley
PRs	bpo-36959: Fix checks for invalid ISO date formats in _strptime #13408

^{Note: these values reflect the state of the issue at the time it was migrated and might not reflect the current state.}

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GitHub fields:

assignee = None
closed_at = None
created_at = <Date 2019-05-18.19:46:43.829>
labels = ['tests', '3.8', 'library']
title = 'ISO date errors in _strptime are jumbled'
updated_at = <Date 2019-05-19.01:10:45.613>
user = 'https://github.com/GPHemsley'

bugs.python.org fields:

activity = <Date 2019-05-19.01:10:45.613>
actor = 'gphemsley'
assignee = 'none'
closed = False
closed_date = None
closer = None
components = ['Library (Lib)', 'Tests']
creation = <Date 2019-05-18.19:46:43.829>
creator = 'gphemsley'
dependencies = []
files = []
hgrepos = []
issue_num = 36959
keywords = ['patch']
message_count = 5.0
messages = ['342810', '342811', '342812', '342816', '342828']
nosy_count = 2.0
nosy_names = ['p-ganssle', 'gphemsley']
pr_nums = ['13408']
priority = 'normal'
resolution = None
stage = 'patch review'
status = 'open'
superseder = None
type = None
url = 'https://bugs.python.org/issue36959'
versions = ['Python 3.8']

[GPHemsley]

This has not been apparent because the tests for this code are not testing what they think they're testing.

[pganssle]

@gphelmsley Can you clarify what you mean by this?

Do you have a minimal reproducing example that shows what's happening and what you are expecting?

[GPHemsley]

I've created a PR that fixes the issue, which I discovered while evaluating the test coverage for _strptime.

Certain scenarios of error messages were never being hit because the cascade was out of order, and the tests were not showing that because they were throwing a different ValueError than the one they were expecting to throw.

[pganssle]

Hm, I was a bit confused by your wording here, because I am able to trigger all the errors just fine even before this PR, but I do think that even though this isn't necessarily fixing inaccurate error messages (all the error messages *are* accurate), you're right that the text of the messages does seem to indicate that the original authors intended a cascade ordering more like the one you've proposed.

I'll give this a more thorough review a bit later, thanks for working on this!

[GPHemsley]

Ah yes, to be clear, I wasn't trying to suggest that the error messages themselves were wrong—just that they weren't triggering when the tests were expecting them to.

Some of the existing tests currently trigger the "unconverted data remains" ValueError from earlier in the method, but because the messages are not checked, that is not immediately obvious. I've also added new tests for additional scenarios that would presumably also be considered invalid, based on the existing ones.

[serhiy-storchaka]

There are two changes in test results:

_strptime('1999 256', '%G %j'):
was "ISO year directive '%G' must be used with the ISO week directive '%V' and a weekday directive ('%A', '%a', '%w', or '%u')."
proposed "Day of the year directive '%j' is not compatible with ISO year directive '%G'. Use '%Y' instead."

_strptime('50 5', '%V %u'):
was "ISO week directive '%V' is incompatible with the year directive '%Y'. Use the ISO year '%G' instead."
proposed "ISO week directive '%V' must be used with the ISO year directive '%G' and a weekday directive ('%A', '%a', '%w', or '%u')."

In both cases the former message refers to a directive that does not appear in the format string.