BaseForm.add_error() misleading exception message when both field and dict provided

[Chris Brand]

This code creates a form and attempts to add an error for a single field to it:

from django import forms
from django.core.exceptions import ValidationError

class FooForm(forms.Form):
    the_field = forms.CharField(max_length=100)

exc = ValidationError({'the_field': 'Something is wrong with the field'})
form = FooForm()

form.add_error('the_field', exc)

It results in the following error message:

TypeError: The argument `field` must be `None` when the `error` argument contains errors for multiple fields.

The reference to "errors for multiple fields" is just wrong - the error in question is for a single field.

For background, in my case the ValidationError is raised by my custom Model.clean(). What I really want to do it to map it from the Model field to the form field (which in general may not have the same name). The error message was quite confusing.

[Jacob Walls]

Good spot, happy to take a PR replacing the reference to "multiple fields" with a mention of a dictionary instead. Incidentally this condition ​isn't tested, so we'll benefit from the addition of a test.

[The5cheduler]

Raised the PR Here:

​https://github.com/django/django/pull/19898

[Jacob Walls]

Thanks for volunteering. I think my triage came in while you were working on a PR, so be sure to read the triage decision: a PR to adjust the exception message is welcome, but it wouldn't be in scope to change the underlying behavior.

[The5cheduler]

Thank you @jacobtylerwalls
I have adjusted the files to update the messaging of the error without updating underlying behavior of the method.