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Benjamin Orimoloye23,328 Points
Why the "form" parameter? It was never used in the function
In the function def username_exists(form, field):, field was used, but form wasn't. Why isn't there?
Nicolas Hampton44,626 Points
The best answer to this is looking at the source code for the "EqualTo" validator: https://github.com/wtforms/wtforms/blob/283b2803206825158834f1828bbf749c129b7c47/src/wtforms/validators.py#L88
The "form" reference that's passed into validators makes it possible to validate based on the value of other fields as well. The perfect example of this is a password and confirm password field, where the validity of one field is dependent on another.
Two things about this source code: the validator is a class, not a function. Don't let this throw you. The call method is basically the same as the function validators we're writing in this class. They're creating a class here to allow you to pass arguments to the init method to customize the validator method.
Another student asked on the forums why EqualTo takes a string instead of a reference to password2, and here you can see why. Internally, EqualTo is using that string as a key for the form parameter!
Hope this helps!
J llama12,629 Points
Kenneth does a pretty bad job of explaining this one here, i have the same question