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time.strftime(arg) fails when arg is not valid format string #4157
Description
Expected Result
In cpython, when arg is not a valid format string, strftime returns arg itself.
>>> import time
>>> time.strftime("%Y")
'2022'
>>> time.strftime("%4Y")
'4Y'
Actual Result
In RustPython, strftime raise panic when arg is not a valid format string
>>>>> import time
>>>>> time.strftime("%Y")
'2022'
>>>>> time.strftime("%4Y")
thread 'main' panicked at 'a Display implementation returned an error unexpectedly: Error', /rustc/4b91a6ea7258a947e59c6522cd5898e7c0a6a88f/library/alloc/src/string.rs:2490:14
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace
Activity
jopemachine commentedon Sep 12, 2022
I think this could be related to the below issue.
chronotope/chrono#815
jopemachine commentedon Oct 4, 2022
Now, I'm waiting for the below PR to be merged and v0.5 to be released.
chronotope/chrono#827
itsankitkp commentedon Jan 26, 2023
Should we add a validation check in fn strptime for supported formats?
This would require a list valid_time_formats = [..] which we can get from https://docs.python.org/3/library/time.html#time.strftime
if validation succeeds then we can continue normal flow otherwise we can return an arg
DimitrisJim commentedon Jan 27, 2023
There's a simpler solution for now, we can use
write!to create the formatted date instead ofto_string, that returns a result which we can handle.Do you want to give it a shot? If so, lmk and I'll assign you.
itsankitkp commentedon Jan 27, 2023
Did some more digging. CPython uses wcsftime for formatting time (if system supports it)
After building latest python 3.12, I got following
Same for Python 3.10 as well
Playing around with wcsftime for same input also yields same result
https://en.cppreference.com/w/c/chrono/wcsftime
Now, as for chrono in rust:
I attempted write!
write!(&mut formatted_time, "{}", instant.format(format.as_str())).unwrap();
It panicked at %4Y (worked at other cases so at least I am not passing it wrong)
I played around chrono too, it doesn't support %4Y at all.
What can be done now is to add exception handling so that rustpython don't panic if there is an exception in chrono.
In case of exception, it can return output as arg. Whenever chrono gets updated, we will get correct results.
Sure, I have raised PR #4474
which includes above solution with a test case
Pending: one test case is failing viz embedded null character
I think null characters should have been handled in earlier layers (not related to change in this PR), I will check this.
fanninpm commentedon Jan 27, 2023
After reading a bit more, it seems that the
"%4Y"format is platform-specific.itsankitkp commentedon Jan 28, 2023
+1 and on compiler version
DimitrisJim commentedon Jan 30, 2023
yeah,
cronoshould be the one handling the platform specific stuff. Our main goal should be to avoid the panic for now and try and return the format as CPython does (i.e they probably remove the leading%on return if I remember correctly last I checked it)itsankitkp commentedon Jan 31, 2023
Yes, current version implements this behavior.
fanninpm commentedon Feb 23, 2023
Did #4530 fix this?
DimitrisJim commentedon Feb 24, 2023
it did go from panicking to simply failing so partially yes? 😄
[-]`time.strftime(arg)` raise panic when arg is not valid format string[/-][+]`time.strftime(arg)` fails when arg is not valid format string[/+]kong-peter commentedon Apr 13, 2025
Hi, new to this project and just reviewing the "good first issue" label and i cannot reproduce the issue!
On my Mac, RustPython 3.13.0 now does:
youknowone commentedon Dec 25, 2025
This is fixed, thank you all!