Flask WTForms: Difference between DataRequired and InputRequired

Short Answer

Unless you have a good reason you should use InputRequired

Why?

Lets look at some notes from the docs/code for DataRequired() :

Note there is a distinction between this and DataRequired in that InputRequired looks that form-input data was provided, and DataRequired looks at the post-coercion data.

and

NOTE this validator used to be called Required but the way it behaved (requiring coerced data, not input data) meant it functioned in a way which was not symmetric to the Optional validator and furthermore caused confusion with certain fields which coerced data to 'falsey' values like 0, Decimal(0), time(0) etc. Unless a very specific reason exists, we recommend using the :class: InputRequired instead.

what does this mean?

In the Form class you will notice two keyword arguments formdata and data. These generally correspond with two methods process and process_formdata. When form data comes in off the wire its not always in the format corresponding to the Field type. A good example of this is the value u'1' being supplied to an IntegerField. This would be bad news if you had a NumberRange validator because u'1' isn't a number.

The primary purpose of the process_formdata method is to prevent this situation by coercing the value into its correct type prior to running validation rules. That is what they are referring to when they say "looks at the post-coercion data"

the problem!

Both InputRequired and DataRequired work the same way specifically the __call__ implementations:

def __call__(self, form, field):    if not field.data or isinstance(field.data, string_types) and not field.data.strip():        if self.message is None:            message = field.gettext('This field is required.')        else:            message = self.message

Certain field types coerce data into Falsey values(0, Decimal(0), etc.). The problem occurs when you have an IntegerField and the form submits a value like '0'. If you apply DataRequired to this it will fail validation. This is because DataRequired will evaluate if not field.data... after coercion where field.data is the Falsey numeric value 0.