convert Len to GroupedMetadata, add MinLen and MaxLen

[samuelcolvin]

Fairly self explanatory, should be backwards compatible.

I need MinLen and MaxLen in pydantic to convert fields arguments to constrains.

[Zac-HD]

This is a good enough idea that I'm embarrassed I didn't think of it myself! Two comments:

• needs to be documented in the README too
• I have a little FUD around interpretation of the name Len when I'm going to use this for eg iterables on which it's an error to call len(). Best option IMO is to keep it and I'll add a note like "implementors may also apply this constraint in cases where len() cannot be used but the number of elements is still meaningful, such as iterators"

[samuelcolvin]

I'll add to the README.

We could call it Length, to make it a bit different to len()? But it doesn't help much.

[adriangb]

How about NumItems?

[Zac-HD]

I'm voting for the existing name, to be clear, just intending to add a one-sentence clarifying note!

[samuelcolvin]

I'm voting for the existing name, to be clear, just intending to add a one-sentence clarifying note!

Agreed.

[samuelcolvin]

This is a good enough idea that I'm embarrassed I didn't think of it myself!

I think we didn't have GroupedMetadata when Len was implemented, so a totally understandable omission.

needs to be documented in the README too

Done.

Please review.

[Zac-HD]

This is a good enough idea that I'm embarrassed I didn't think of it myself!

I think we didn't have GroupedMetadata when Len was implemented, so a totally understandable omission.

Yeah, but I didn't notice when we added GroupedMetadata or even MinLen and MaxLen - it's the missed "should we apply this refactoring anywhere else" that I'm practicing. Not a big deal though 😸