Question

I have subclassed a text-field form field in Django to create my own custom widget for a field. I was wondering if it’s possible to check if all other fields of the form are valid (I want its server side behavior to vary based on the validation of other fields)

See comment

Something like:

class CustomField(TextInput):
  def __init__(self, *args, **kwargs):
    ...
    super(CustomField, self).__init__(*args, **kwargs)

  input_type = 'hidden'

  def value_from_datadict(self, data, files, name):
    aws_file_key = data.get(name, None)
    _media_bucket = boto.connect_s3(settings.AWS_ACCESS_KEY_ID,
                                       settings.AWS_SECRET_ACCESS_KEY)\
                            .lookup(settings.AWS_MEDIA_STORAGE_BUCKET_NAME)

    try:
      key = _media_bucket.get_key(aws_file_key)
    except:
      print 'Failed to get key.'
      key = None

    if key and aws_file_key:
      fh = tempfile.TemporaryFile()
      key.get_contents_to_file(fh)
      fh.seek(0)
      files = SimpleUploadedFile(key.name, fh.read())
      ### IF FORM IS VALID DELETE KEY, OTHERWISE, KEEP IT.
      if code_to_check_if_valid:
        _media_bucket.delete_key(key)
      fh.close()
      return files
      ...... etc.

Answer 1

If you want to validate a certain field depending on the values of other fields, you need to to it at the form level and overwrite the field’s clean method. Here’s the docs on the subject – they are very good.

class CustomForm(forms.Form):
    custom_field = CustomField()

    def clean(self):
        cleaned_data = super(CustomForm, self).clean()
        custom_field = cleaned_data.get("custom_field")
        ...

If you look at the flow of how forms are validated, you will see that the clean method is run if all the other fields validate independently, so at this stage, the form can be considered valid:

These methods are run in the order given above, one field at a time. That is, for each field in the form (in the order they are declared in the form definition), the Field.clean() method (or its override) is run, then clean_<fieldname>(). Finally, once those two methods are run for every field, the Form.clean() method, or its override, is executed.

The final clean method is actually run regardless of if there’s an error so you have to iterate through the cleaned_data to make sure there are no errors

The clean() method for the Form class or subclass is always run. If that method raises a ValidationError, cleaned_data will be an empty dictionary.

The previous paragraph means that if you are overriding Form.clean(), you should iterate through self.cleaned_data.items(), possibly considering the _errors dictionary attribute on the form as well. In this way, you will already know which fields have passed their individual validation requirements.