Allow overriding of error message for each newforms Field type

Sure -- good idea. Could you provide a patch, along with unit tests that confirm this behavior for every field?

I tried out some code changes (and tests) for implementing a patch. One key question I would like to see answered is how this change should affect the behaviour of RegexField? and its subclasses, say URLField. The tests indicate that these classes expect two kind of error messages depending on the nature of the error.

Consider the first kind of error message, raised when the value of the field doesn't fit a given pattern.

>>> f = URLField()
>>> f.clean('foo')
Traceback (most recent call last):
...
ValidationError: [u'Enter a valid URL.']

The error message displayed is the one used by URLField to override the default provided by RegexField?.

Now consider the second scenario where the field is required but turns up empty.

>>> f.clean('')
Traceback (most recent call last):
...
ValidationError: [u'This field is required.']

The error raised in this case is different (the default error defined in the base 'Field' class is used)

If a URLField object is created with a custom error_message, should it still fall back to the default message if value turns out to be empty? Likewise how should a CharField? object with a custom error_message behave in such a case?

One possible solution would be to insist that custom error messages take precedence and hence will be always used if present. I don't mind this but I can see why others could have problems with this. Another approach would be to "collect" errors applicable to each field and display them together (a la Rails model validation).

The latter implementation would result in something similar to this:

>>> f.clean('')
Traceback (most recent call last):
...
ValidationError: [u'This field is required.', u'Enter a valid URL.']

See also #4063, which proposes a more elegant solution.

#4063 marked as a duplicate.

Need to decide what to do with Fields that can raise more than one Error.

I suggest to use dictionary when multiple errors could occur.

Ok, here you go: a working patch. Just needs documentation and tests...

fixed EmailField behaviour and made sure current tests still pass (ErrorList.__repr__ now uses force_unicode)

For a quick summary of how it works:

1. each Field instance gets an error_messages dictionary built from the default_error_messages in it's class (and base classes, recursively)

2. a user can pass in their own error_messages dictionary which .update()s the error_messages dictionary for that instance

>>> from django.newforms import *
>>> d = DecimalField(error_messages={'invalid':'Mr T says, "Enter a valid number, foo!"'})
>>> d.clean('jibberjabber')

Traceback (most recent call last):
  File "<pyshell#72>", line 1, in -toplevel-
    d.clean('jibberjabber')
  File "[...]\django\newforms\fields.py", line 237, in clean
    raise ValidationError(self.error_messages['invalid'])
ValidationError: [u'Mr T says, "Enter a valid number, foo!"']

with full tests and docs

In reality, noone else was going to do docs or tests. So here they are.

brought patch up to date, but tests still failing for python 2.3 due to non-evaluation of lazy objects when printing out ErrorList?.

removed my changes from testcases.py I made during testing

(In [6625]) Fixed #3457 -- Allow overridding of error messages for newforms Fields.

Sorry that I forgot to mention it in the commit message, but many thanks to SmileyChris for the bulk of the work done on this patch.