Non boolean values for field_flags

[Sohalt]

This allows to specify a function returning a dictionary for field_flags.
This allows validators to set flags which are supported directly in HTML for client side validation, such as minlength, which then get rendered appropriately by the widgets supporting that attribute.

Fixes #406

[Sohalt]

I haven't written any documentation yet. Also this could probably use some more tests. And ideally the Regexp validator would set the pattern attribute, but I am not sure if you can just set the Python pattern verbatim, because JS might support a different subset of regex.

[Sohalt]

@davidism the stylecheck fails because some lines are too long (> 80 characters). The docs say this is not a hard limit and I see other lines in test_fields.py with more than 80 characters that don't get flagged. Can you please have a look and tell me how to best resolve this? The affected lines are html literals for testing, for which it doesn't make much sense to split them over multiple lines imho.

[davidism]

I believe Flake8 is configure to allow what Black allows: 88 characters max. Literals should be split with implicit continuations:

value = (
    "This is a"
    " long sentance."
)

[Sohalt]

For some reason coveralls fails. I don't think this is an issue with my code.

[Sohalt]

Ok, it seems the coveralls API failed. Simply restarting the job worked… Can you have a look at the code now @davidism and tell me any issues? If everything is ok code wise, I'll write some documentation.

[davidism]

When we have time, I or another maintainer will handle this.

[Sohalt]

Thank you! I just wanted to make sure this is still on your radar.

[whb07]

@Sohalt I've been afk for a bit on this project 😞, so could you tell me why you chose to go with a function call rather than a property/attribute of sorts?

[Sohalt]

It's been a while, so I don't exactly remember the reason. But I think if I remember correctly I originally tried implementing it with a dictionary, which failed however for my specific use case. I think using the lambda was required to delay evaluation, because some information was not yet available when setting the flags.

[azmeuk]

I switched the tests to the pytest syntax and rebased this branch.

I can understand the use of field_flags being a function, to dynamically create a dict, though this is not the only way to do that.
I think I would like it better if field_flags was a dict initialized in validators __init__.

Then with some documentation I think this PR would look good.

[azmeuk]

This seems good to me now. However I don't know if this patch should target v3.x or v2.x.
Since it does not break anything, I think both are possible.

[azmeuk]

Depending on which branch this PR lands, we should explicit the next version.