Simplified length limited string

[samuelcolvin]

I often want constr(max_length=XXX, strip_whitespace=True). Could we add a shortcut for this that worked better with mypy?

E.g. something like

class Foobar(BaseModel):
    name: StrLimit[63]

This would be equivalent to constr(max_length=63, strip_whitespace=True), e.g a field with a max length of 63, no min limit, strip_whitespace enabled.

The name StrLimit is debatable. I think using ConStr might be confusing, but perhaps would make more sense?

We could also allow StrLimit[10, 63] which would be equivalent to constr(min_length=10, max_length=63, strip_whitespace=True). However this would not be very pythonic in that the first argument meaning would change. Also I find myself wanting this very rarely.

thoughts?

[layday]

You can't really do this with typing in Python, I don't think. The issue here is that to parametrise ('subscribe') a class for type checkers, it needs to be made generic. But a generic type can only be parametrised with other types - 63 is an instance of int and would be invalid. The only exception to this is Annotated.

[JacobHayes]

Perhaps this would be possible/clean/generic with PR #2147 and FR #2129 (working prototype)?

class Foobar(BaseModel):
    name: Annotated[str, MaxLen(63), Trim()]

[antonagestam]

@layday StrLimit doesn't need to be a class, it can be a subscribable object.

@samuelcolvin If we want to support both upper and lower bounds, perhaps StrLimit[:63] makes sense as syntax?

[samuelcolvin]

Perhaps this would be possible/clean/generic with PR #2147 and FR #2129 (working prototype)?

class Foobar(BaseModel):
    name: Annotated[str, MaxLen(63), Trim()]

This should work now, although we need to add Trim().

@layday StrLimit doesn't need to be a class, it can be a subscribable object.

@samuelcolvin If we want to support both upper and lower bounds, perhaps StrLimit[:63] makes sense as syntax?

We had this in annotated-types but removed it because the upper value of a slice is max-exclusive which makes no sense, see annotated-types/annotated-types#23.

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The point with this feature is to keep it simple and cover the most common case which is having an upper length limit - e.g. to be equivalent-to/compatible-with varchar() in sql.

[adriangb]

I don't think this is necessary any more.