Multiple fields support

[azmeuk]

I could not find it clearly written on the specs, but HTML allows an input file to have several values. For instance this is valid HTML:

<input type="text" name="foo" value="bar">
<input type="text" name="foo" value="baz">

However, wtforms does not support this, and always consider the first item of the request for a given name (here bar).
https://github.com/wtforms/wtforms/blob/9f9146c8ee2cb2b2e7b3d9bc09ac7e433fd55bb0/src/wtforms/fields/core.py#L362-L363

wtforms provides an alternative to manage several times the same input, with FieldList. However FieldList have some differences:

• the input have an incremental suffix, so this makes JS reordering of the fields more complicated for instance
• resizing of the data list must be done with append_entry and pop_entry, users cannot directly delete a field in the middle of the list
• it allows to nest FormField and make complex forms.

I think wtforms could support multiple fields. For instance we could add a multiple field in Fields with a default value at False, matching the current behavior. If set to True field.data would return a list instead of a single value. Validators should validate each individual values.

This could help with the validators handle one single value part of #662

[davidism]

I've thought about this before, but it feels like we're getting to the point of requiring a huge redesign of the entire library. There are nuances to accepting both single and list, validating each item or the whole list, handling a list of same-named fields where only some where filled out, applying required appropriately, etc. This is partially what I commented about in #662, because the current way we handle data is inconsistent and incomplete compared to what's possible in HTML.