Allow durationfield to be represented as ISO-8601

[sevdog]

As of now for date and time field there is a config to specify the output format which allows to use the ISO-8601 format (which is also supported by django in its parsers).
Whereas DurationField are always formatted using django base method duration_string. There is no way to configure a whole project to use duration_iso_string instead to have durations always formatted as ISO-8601.

Is it true that you can override the DurationField to use that format, but when you have a dependency which uses this kind of field this option would lead to overwrite that code, bringing maintenance problems.

It would be cool if there could be a settings option to switch between base and ISO representation of durations.

[tomchristie]

Could you start by giving an example of what format you'd want to see?

date/times and durations are different types of things, and it's not clear to me that ISO-8601 is relevant here.

[sevdog]

@tomchristie I am referencing to the ISO-8601 which is also supported by parse_duration method:

e.g. P4DT1H15M20S which is equivalent to 4 1:15:20

As of now the DurationField only allows the latter representation (4 1:15:20) in to_representation method:

django-rest-framework/rest_framework/fields.py

Lines 1404 to 1405 in 2506d0b

	def to_representation(self, value):
	return duration_string(value)

Allowing to use ISO representation for duration (via django.utils.duration.duration_iso_string) would improve interoperability with other languages which may interact with provided API since this is standard format thus is easier to have a ready-to-use parser for it.

[tomchristie]

Ah I see it now... https://en.wikipedia.org/wiki/ISO_8601#Durations

Since that's in line with the existing DateField and TimeField I think we'd accept a pull request adding that functionality.

A good place for someone to start on this would be proposing what the documentation would look like after the change, since that'd be a really good point of review. Then adding the functionality and test cases to match up with the updated docs.

[sevdog]

@tomchristie I have opened a PR #8532 (in draft) with the updated doc.

I have tried to keep it aligned with date-time fields, I'm waiting for your opinion.