Get textField value and make text if exist in the dataBase (forms, Python)

I have an interface including an inputText and submit button, I would like to test (forms.py) if the value entered already exists in the data base when I click to save the key entered in the textField. I am still new on Python, thank you for your help.

def save(self, commit=True):
    # validate if legacy key exist
    key_value = self.data.get('key_id')
    try:
        key_request = self.key_id.filter(key_value=self.key)
        if key_request.exists():
            raise Exception(_("key already exists"))
    except Academic.DoesNotExist:
        pass