dynamic widget parameters for CharField

[matthewhegarty]

Closes #1485

Dynamic widget parameters for CharField allows for storing 'None' in xlsx as empty string.

When Resource is created, if the underlying field is CharField or Subclass, and if it allows empty strings, then create kwargs which get passed to the widget, which allow for storing of empty strings.

See also #1623

Acceptance Criteria

• Added integration tests
• Manual testing of the steps defined in the issue

[pokken-magic]

I think I'm in alignment here.

Long term I think we may want to have some dialog about formats handling of nulls and try to structure that a bit better, but I think you're going in the right direction.

[pokken-magic]

How will this behave if null and blank are both true? My naive reading is both nulls and blanks will end up as empty strings and come back in as empty strings, and this is universal across all formats (whether they support nulls or not).

I think this is a reasonable assumption, and blank+null handling is problematic as heck in Django already.

From what I am reading Django really discourages using both blank and null, and prefers blanks for text type fields. So we're following best practice if I"m reading right.

-- Basically, if the field allows blanks, we treat all nulls as blanks on export, and empty fields/cells as blanks on import. --

[matthewhegarty]

Hi Ryan, thanks for reviewing, much appreciated.

How will this behave if null and blank are both true?

'blank' is the overriding setting - so it will store None as empty string.

Basically, if the field allows blanks, we treat all nulls as blanks on export, and empty fields/cells as blanks on import.

Yes, this is the nub of it. This change only affects imports. There are other changes planned to make exports export either string or raw type.

Users can override the behaviour added in this PR by declaring their own Field, e.g. this declaration in core/admin.py will cause the original error to re-occur.

    author_email = fields.Field(
        attribute="author_email",
        widget=widgets.CharWidget(allow_blank=False, coerce_to_string=False))