Flag attribute as required / mandatory

[elgalu]

Hi, thanks for this very useful library!

How do I flag attribute as required / mandatory?

I have this:

from traitlets import HasTraits, Unicode, TraitError, validate

class MyS3Class(HasTraits):
    s3_bucket_name = Unicode()

    @validate('s3_bucket_name')
    def _check(self, proposal):
        raise TraitError("this doesn't trigger unless explicit non empty string is set")

But @validate('s3_bucket_name') doesn't get triggered if the user doesn't pass a value.

object = MyS3Class()
# FAIL: doesn't trigger _check()

object = MyS3Class(s3_bucket_name='')
# FAIL: doesn't trigger _check()

object = MyS3Class(s3_bucket_name=' ')
# OK: properly triggers _check()

[elgalu]

For now I will "manually" trigger validations via:

class MyS3Class(HasTraits):
    def __init__(self, **kwargs):
        super(MyS3Class, self).__init__(**kwargs)
        self.s3_bucket_name = self.s3_bucket_name

[rmorshea]

@elgalu I would do the following:

class MyS3Class(HasTraits):
    
    s3_bucket_name = Unicode()

    @validate("s3_bucket_name")
    def _validate_s3_bucket_name(self, proposal):
        if not proposal.value:
            raise ValueError("S3 bucket name cannot be empty.")

    def __init__(self, s3_bucket_name, **kwargs):
        super().__init__(**kwargs)
        self.s3_bucket_name = s3_bucket_name

This both enforces the fact that the trait must be provided when instantiated and that it not be empty.