Bidirectional form data filters

[ThatMorneGuy]

I use a custom filter to add the currency prefix to a FloatField to make it more user friendly. This also adds the currency prefix to the value in my edit form.

from app import app

@app.template_filter()
def format_currency(value:float, currency:str=None):
    if currency == 'AUD':
        return "AU${:,.2f}".format(value)
    else:
        return "${:,.2f}".format(value)


from app._filters import format_currency

class ContractForm(Form):
    cost = FloatField('Cost', id='contract_cost', default=0.00, filters=[format_currency])

Is it possible (and how would one implement) to use a filter to remove the currency prefix from the field data on submit before form validation?

I have gone through documentation and searched forums but haven't found a solution.

[t0hi0]

@ThatMorneGuy, hello! You can create a custom field that inherits from FloatField and override the process_formdata method with your clear prefix logic.

[azmeuk]

If I understand correctly, the issue you are describing is that the data is validated after being transformed by filters, for instance:

>>> import wtforms
>>> def format_currency(value:float, currency:str=None):
...     return "${:,.2f}".format(value)
>>> class ContractForm(wtforms.Form):
...     cost = wtforms.FloatField(filters=[format_currency], validators=[wtforms.validators.NumberRange(1, 10)])
>>> f = ContractForm(cost=2.00)
>>> f.validate()
Traceback (most recent call last):
  File ".../validators.py", line 205, in __call__
    and not math.isnan(data)
            ^^^^^^^^^^^^^^^^
TypeError: must be real number, not str

And you would like a way to transform the data after it is being validated. Is this what you mean?

[azmeuk]

I actually misunderstood the issue.

So you would like:

• a mechanism that formats field.data for display only, without touching the value used by validation or stored on the form;
• and a mirrored mechanism that strips the formatting from the submitted string before process_formdata coerces it.

As my previous comment showed, filters are not suitable for either: they apply between process_formdata and validation, mutating field.data itself. So they alter validation input (your example) and run too late to clean up the submitted string before coercion.